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  • Theoretical, Physical and Computational Chemistry  (1,576)
  • Neurologie
  • Theoretische Physik
  • Messtechnik
  • Strömungsmechanik
  • Wiley-Blackwell  (1,576)
  • Springer  (5)
  • Oxford University Press
  • American Institute of Physics (AIP)
  • 1995-1999  (1,581)
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  • Springer  (5)
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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Der Anaesthesist 48 (1999), S. 630-638 
    ISSN: 1432-055X
    Keywords: Schlüsselwörter Intrakranielle Druckmessung ; Messgenauigkeit ; Intrakranielle Drucksonden ; Messtechnik ; Compliance ; Key words ICP monitoring ; Measurement accuracy ; ICP probes ; Compliance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Description / Table of Contents: Abstract Goal: Intracranial pressure (ICP) monitoring has a key role within the neuromonitoring, although ICP does not monitor processes of the central neuron directly and only with delay. One of the important factors in ICP monitoring is measurement accuracy. For a better understanding of ICP probes and their differences, the function and principles of intracranial pressure transducers should be evaluated from a technical point of view. Method: The principles of ICP measurement were analyzed and compared. Practical applications of these principles were examined and examples of different ICP probes were discussed regarding their mode of pressure transformation. The technical advances of ICP monitoring were analyzed. Results: Since LUNDBERG, a variety of different types of transducers has been developed. Ventricular ICP monitoring has been supplemented by extradural and intraparenchymatous probes. An increasing miniaturization of the transducers has emerged. Additionally, fiberoptic systems have been developed. Latest developments include multifunctional ICP probes. So far, the main problem of most types of transducers consists in the inability to assess measurement accuracy of a probe during the period of patient monitoring. Conclusion: ICP probes should be tested better for correct function by the manufacturer prior to sale. External controls of the measurement accuracy should be performed more frequently to ensure constant quality. Future ICP transducers have still to be more cost- effective.
    Notes: Zusammenfassung Fragestellung: Die Messung des intrakraniellen Drucks hat eine Schlüsselstellung innerhalb des Neuromonitorings erhalten, obwohl durch den Hirndruck die eigentlichen metabolischen Prozesse am zentralen Neuron nicht direkt und auch nur zeitlich verzögert registriert werden können. Voraussetzung für eine zuverlässige Hirndruckmessung sind spezifische Eigenschaften von Hirndrucksonden, die im folgenden untersucht werden sollen. Diese sollen dabei aus messtechnischer Sicht analysiert und kategorisiert werden, um ein besseres Verständnis für deren unterschiedliche Funktionsweise und Messeigenschaften zu erhalten. Methodik: Es werden die verschiedenen Messprinzipien, die bislang zur Anwendung kamen, dargelegt. Es wird aufgezeigt, wie diese Prinzipien praktisch genutzt werden. Probleme der Messgenauigkeit werden aus messtechnischer und klinischer Sicht erörtert. Ergebnisse: Es wurden seit Lundberg eine Vielzahl verschiedener Transducertypen entwickelt. Die ventrikuläre Hirndruckmessung wurde ergänzt durch epidurale und intraparenchymatöse Sonden. Eine zunehmende Miniaturisierung der Transducer und eine Verbesserung der Messtechnik hat eingesetzt. Zusätzlich wurden fiberoptische Systeme entwickelt. Neueste Entwicklungen zielen auf Multifunktionssonden ab, die zusätzlich zum intrakraniellen Druck auch die Hirntemperatur und weitere Parameter gleichzeitig messen können. Hauptproblem vieler der bisherigen Sonden ist das Fehlen einer direkten Kontrolle des Messverhaltens während des klinischen Einsatzes. Schlussfolgerungen: Sonden sollten vor Gebrauch vom Hersteller noch besser auf Funktionsfähigkeit überprüft und entsprechend ausgewiesen werden. Externe Kontrollen zur Überprüfung des Messverhaltens von intrakraniellen Drucksonden sollten zusätzlich verstärkt durchgeführt werden, um die Qualitätssicherung zu verbessern. Zukünftige intrakranielle Drucksonden müssen noch kostengünstiger werden.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron affinities (EA) of B, Al, and Ga are determined accurately at the CCSD(T), QCISD(T), and B3LYP levels of theory. The failure of the QCISD(T) and B3LYP methods reported by Jursic is attributed to his treating the wrong state of the ion.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 285-286, 1998
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  • 3
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Basis-set expansions of the form ∑kCk(1±Pyz)ψk(x, y, z), where ψk(x, y, z)=xnkymkzlke-∑13(pkrtr+qkrtr2) is a generalized six-parameter Hylleraas-Gaussian (HG) ansatz expressed in perimetric coordinates (t1=x, t2=y, t3=z), are proposed in the description of atomic systems entering the helium isoelectronic sequence. Variational nonrelativistic calculations for the ground 11S state energy of H- plus ground 11S and 23S state energies of He, Li+, Be2+, and B3+, along with some wave-function properties associated with these states, are presented. Comparisons are made between the accuracy of minimal HG expansions for atomic ground states (from Z=1 to Z=10) with that of other, previously explored basis sets. The convergence in energy and wave-function properties is analyzed under a systematic increase of the expansion lengths ranging from 12 to 150 terms. The choice of parameters is based on pseudorandom sequences spanned on variationally optimized intervals [Plr, Pur] and [Qlr, Qur] (r=1, 2, 3) that generate all pkr's and qkr's, respectively, according to random tempering formulas for selected low-order HG expansions. The latter deliver core functions over which a systematic increase of basis sets is performed. Energies obtained through the highest expansions agree with the exact results, or best estimated values available in the literature, within an accuracy of 10-8-10-7 Hartrees.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 287-300, 1998
    Additional Material: 6 Tab.
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  • 4
    ISSN: 0020-7608
    Keywords: LaC3n+ ; structure ; vibrational frequency ; density functional theory ; Gaussian 92/DFT ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: LaC3n+ (n=0, 1, 2) clusters have been studied using B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional method. The basis set is Dunning/Huzinaga valence double zeta for carbon and [2s2p2d] for lanthanum, denoted LANL1DZ. Four isomers are presented for each cluster; two of them are edge binding isomers with C2v symmetry, the other two are linear chains with C∞v symmetry. Meanwhile, two spin states for each isomer, that is, singlet and triplet for LaC3+, doublet and quartet for LaC3 and LaC32+, respectively, are also considered. Geometries, vibrational frequencies, infrared intensities, and other quantities are reported and discussed. The results indicate that at some spin states; the C2v symmetry isomers are the dominant structures, while for the other spin states, linear isomers are energetically favored.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 301-307, 1998
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  • 5
    ISSN: 0020-7608
    Keywords: Sulfine ; sulfene ; Diels-Alder reaction ; semiempirical method ; AM1 ; PM3 ; transition structure ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: AM1 and PM3 computations show that thiocarbonyl S-oxide and thiocarbonyl S,S-Dioxide undergo Diels-Alder cycloadditions with cyclopentadiene and anthracene to form thiabicyclic adducts through asynchronous transition structures (TSs) with C(DOUBLE BOND)S π* involving in the reaction at a very early stage. Calculated activation barriers indicate that the dienophilicity of these heterocumulenes decrease gradually with progressive addition of oxygen atom on thiocarbonyl sulfur, in reasonable agreement with experimental observations. Frontier Molecular Orbital (FMO) and deformation energy analyses reveal that the above trend is due to gradual destabilization of lowest unoccupied molecular orbital (LUMO) of the dienophile and increase of deformation energy of both diene and dienophile with increase of oxygen atoms around sulfur. Analysis of bond orders and TS geometries show that the TSs are neither “early” nor “late.” Chlorine substitution on these heterocumulenes does not seem to increase their reactivity contrary to expectations. The reactions of monosubstituted sulfines with cyclopentadiene pass through four very closely lying TSs and stereoselectively form four stereoisomeric products. For the above reason, the computed barriers show a mixed trend although the relative exothermicity of these reactions are in order.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 309-322, 1998
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  • 6
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This is the first in a series of three articles which aimed to derive the matrix elements of the U(2n) generators in a multishell spin-orbit basis. This is a basis appropriate to many-electron systems which have a natural partitioning of the orbital space and where also spin-dependent terms are included in the Hamiltonian. The method is based on a new spin-dependent unitary group approach to the many-electron correlation problem due to Gould and Paldus [M. D. Gould and J. Paldus, J. Chem. Phys. 92, 7394 (1990)]. In this approach, the matrix elements of the U(2n) generators in the U(n) × U(2)-adapted electronic Gelfand basis are determined by the matrix elements of a single U(n) adjoint tensor operator called the del-operator, denoted by Δji (1 ≤ i, j ≤ n). Δ or del is a polynomial of degree two in the U(n) matrix E = [Eji]. The approach of Gould and Paldus is based on the transformation properties of the U(2n) generators as an adjoint tensor operator of U(n) × U(2) and application of the Wigner-Eckart theorem. Hence, to generalize this approach, we need to obtain formulas for the complete set of adjoint coupling coefficients for the two-shell composite Gelfand-Paldus basis. The nonzero shift coefficients are uniquely determined and may be evaluated by the methods of Gould et al. [see the above reference]. In this article, we define zero-shift adjoint coupling coefficients for the two-shell composite Gelfand-Paldus basis which are appropriate to the many-electron problem. By definition, these are proportional to the corresponding two-shell del-operator matrix elements, and it is shown that the Racah factorization lemma applies. Formulas for these coefficients are then obtained by application of the Racah factorization lemma. The zero-shift adjoint reduced Wigner coefficients required for this procedure are evaluated first. All these coefficients are needed later for the multishell case, which leads directly to the two-shell del-operator matrix elements. Finally, we discuss an application to charge and spin densities in a two-shell molecular system.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 323-343, 1998
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  • 7
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This is the second in a series of articles whose ultimate goal is the evaluation of the matrix elements (MEs) of the U(2n) generators in a multishell spin-orbit basis. This extends the existing unitary group approach to spin-dependent configuration interaction (CI) and many-body perturbation theory calculations on molecules to systems where there is a natural partitioning of the electronic orbital space. As a necessary preliminary to obtaining the U(2n) generator MEs in a multishell spin-orbit basis, we must obtain a complete set of adjoint coupling coefficients for the two-shell composite Gelfand-Paldus basis. The zero-shift coefficients were obtained in the first article of the series. In this article, we evaluate the nonzero shift adjoint coupling coefficients for the two-shell composite Gelfand-Paldus basis. We then demonstrate that the one-shell versions of these coefficients may be obtained by taking the Gelfand-Tsetlin limit of the two-shell formulas. These coefficients, together with the zero-shift types, then enable us to write down formulas for the U(2n) generator matrix elements in a two-shell spin-orbit basis. Ultimately, the results of the series may be used to determine the many-electron density matrices for a partitioned system.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 345-363, 1998
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  • 8
    ISSN: 0020-7608
    Keywords: fullerene derivatives ; C28 cluster molecule ; C28X4 (X=F, Cl, Br, and I) ; PM3 semiempirical molecular orbital method ; 1,3,5,7-tetrahaloadamantane ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Based on the basic theory of C28 cluster molecule proven by H. W. Kroto and the research findings of C28's derivative such as Ti@C28* and Mg@C28, proven by T. Guo, B. I. Dunlap, O. D. Haberlen, and others, we examine the two series fullerene derivatives, C28H4 and C28X4 cluster molecules, which are formed by the skeleton of C28 cluster molecule. In this work, we not only prove that C28 cluster molecule belongs to the Td symmetry structure and its ground state is 5A2 open-shell with four unpaired electrons, but also find that C28 can easily react with single valence electron atoms, like hydrogen atom and halogen atoms, to be formed to stable fullerene derivatives, C28H4 and C28X4 cluster molecules (X=F, Cl, Br, I). The PM3 semiempirical molecular orbital method from G94W and Hyperchem program packages were applied very well in these fullerene derivatives. According to the results presented herein, we obtain the structures of geometrical optimization, ionization potential energy gap, heat of formation, atomization energy, and vibration frequency data of the C28H4 and C28X4 cluster molecules. The above calculation data confirm that these unknown fullerene derivatives are stable molecules; the stable behavior resembles the 1,3,5,7-tetrahaloadamantane molecules. It is quite possible that they can be synthesized experimentally in the near future.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 187-197, 1998
    Additional Material: 9 Ill.
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  • 9
    ISSN: 0020-7608
    Keywords: coupled-cluster theory ; electron correlation effects ; molecular orbital choice ; reference determinant choice ; quasi-degenerate states ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The impact of the choice of molecular orbital sets on the results of the valence-universal coupled cluster method involving up to three-body amplitudes (VU-CCSDT) was studied for the H4 model. This model offers a straightforward way of representing all possible symmetry-adapted orbitals. Moreover, the degree of quasi-degeneracy of its lowest 1A1 states can be varied over a wide range by changing its geometry. Calculations were performed both for 13 sets of standard quantum chemical orbitals and for a vast variety of nonstandard orbital sets defined by nodes of a two-dimensional orbital grid. The performance of various standard orbital sets in VU-CCSDT calculations is compared. It is also documented that for every quasi-degeneracy region there exist nonstandard orbital sets which allow one to obtain more accurate VU-CCSDT energies than the standard orbital sets. In an attempt to provide a general interpretation for some of the alternative orbital sets, we defined a set of orbitals which maximize the proximity of the model and target spaces - maximum proximity orbitals (MPO). It is demonstrated that outside the strong quasi-degeneracy region the energies obtained for the VU-CCSDT approach based on the MPOs are more accurate than for the standard orbital sets.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 221-237, 1998
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  • 10
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The CNM (characterization of normal modes) method for extracting chemical information out of vibrational spectra is tested for vibrational spectra of molecules with relatively strong or relatively weak coupling between internal vibrational modes. Symmetry, parameter set stability, and frequency uncertainty tests are applied to check whether internal vibrational modes, internal mode frequencies, and amplitudes Anμ comply with symmetry, are independent of the set of internal parameters ζn used to describe molecular geometry or fulfill a Lorentzian correlation between amplitudes Anμ and frequency differences Δωnμ=ωn-ωμ. In all cases considered, amplitudes Anμ based on adiabatic internal modes and mass or force constant matrices as metric O are superior to any other definition of amplitude. They represent the basic elements of the new CNM method that leads to chemically reasonable results and presents a new way of extracting chemical information out of vibrational spectra. A number of deficiencies of the potential energy distribution (PED) analysis is discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 41-55, 1998
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  • 11
    ISSN: 0020-7608
    Keywords: linear operator ; *-algebra ; general topology ; asymptotic analysis ; chemical network systems ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Part IV of this series consists of two complementary subparts devoted to attain the following two goals: (i) By shifting from the previous setting of the Banach algebra B(B)=B(B, B) to a broader setting of the space B(X, B) of all bounded linear operators from a normed space X to a Banach space B, we extend our previous theoretical framework to incorporate part of the theory of additive correlation involving the Asymptotic Linearity Theorems, which have been developed for a study of correlation between structure and properties in molecules having many identical moieties, especially in macromolecules having repeating units. (ii) By reverting our focus to the special algebra B(H) with H being a Hilbert space, we develop a theorem which is useful for a structural analysis of spectral symmetry of linear operators representing physico-chemical network systems.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 57-69, 1998
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: bosonic excitation operator ; coherent state ; Gaussian overlap ; time-dependent variational principle ; Hamilton's equation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New excitation operators which have perfect bosonic symmetry are constructed for many-electron wave functions by regarding the system of many electrons as that of many species of bosons. Any electronic configurations can be generated by the new bosonic “void” operators. A coherent state is constructed with the bosonic operators and is adopted as a trial function for the time-dependent variational principle. The equation of motion which has exactly the same form as Hamilton's equation in classical mechanics is obtained with the complex variational parameters, the number of which is equal to the number of electrons.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 71-75, 1998
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  • 13
    ISSN: 0020-7608
    Keywords: density functional theory ; long-range behavior ; electron density ; Kohn-Sham ; orbital energy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total energy, internuclear distance, vertical ionization potential, and electron density function were determined for the hydrogen molecule (H2) with a configuration interaction calculation, a Hartree-Fock calculation, all 22 density functional theory (DFT) methods built into Gaussian 92/DFT, and the Davidson and Jones natural orbital expansion of the Kolos and Roothaan wave function. These data were used to explore the performance of DFT methods in modeling the chemical bond. We noted that although the vertical ionization potentials suggested by the highest occupied molecular orbital energies of DFT methods are generally quite poor, the medium-range bahavior of the electron density from DFT methods is close to that of more accurately determined electron densities. Accordingly, we propose a method of finding an approximate vertical ionization potential for a density functional calculation from the medium-range behavior of the electron density. We explain why the density functionals' electron densities decay in a manner different from that which their orbital energies lead us to expect and explore the relationship between the errors in the exchange-correlation potentials, electron densities, and the orbital energies.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 541-550, 1998
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: density functional ; exchange-correlation ; energy density ; Hooke's atom ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Application of the virial theorem to the interelectronic Coulomb repulsion shows that the virial of the exchange potential yields the exchange energy. However, the virial of the correlation potential does not yield the correlation energy. We have recently constructed a “hypercorrelated” potential whose virial is the correlation energy. We apply these ideas to a system which contains two interacting electrons in an external harmonic potential, Hooke's atom. This system can be solved analytically for a set of spring constants and numerically for any spring constant. By inverting the Kohn-Sham equations, the exact exchange and correlation potentials can be found. These exact values are compared with several popular approximate functionals, namely local spin density (LSD), Perdew, Burke, and Ernzerhof (PBE), and Becke and Lee-Yang-Parr (BLYP). We illustrate our results for two values of the spring constant. At a moderate value, the density is comparable to the He atom, while for a low spring constant, we explore extremely low densities.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 533-540, 1998
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: density functional theory ; electron localization ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using some relationships derived from the density functional version of the virial theorem, an approximated kinetic energy density is constructed. This is used to implement a modified version of the electron localization function (ELF). Its principal differences with the original ELF are that it does not depend on the orbitals and it reduces to zero when the distance r to the nucleus goes to 0 or infinity. Otherwise, it reproduces all the properties of the ELF function. However, the conceptual interpretation of ELF as coming from the pair probability is lost.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 559-565, 1998
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: linear polyene ; T1 state ; wave function ; CIS ; CASSCF ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Configuration interaction singles (CIS) calculations of the planar T1 state of hexatriene, octatetraene, decapentaene, and dodecahexaene showed that the (HOMO-i)→(LUMO+i) type single excitations contribute significantly. This is attributed to the similarity of the HOMO, LUMO, and HOMO-i, LUMO+i overlap densities which stem from nodal properties of the π molecular orbitals. CASSCF calculations for hexatriene and octatetraene also showed remarkable contributions of the (HOMO-i)→(LUMO+i) type singly excited configurations.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 101-106, 1998
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The separation of radial electronic and nuclear motions is discussed with special reference to high Rydberg states of molecules. An inverse separation is obtained when the rapid nuclear motion instantaneously adjusts itself to the position of the Rydberg electron. The electron moves in the potential averaged over the position of the nuclei (and their valence electrons). This inverse separation is useful when ωn3 〉 1, where ω is the spacing of nuclear energy states (in au) and n is the principal quantum number of the Rydberg electron whose orbital period increases as n3. The inverse Born-Oppenheimer separation can break down owing to the finite kinetic energy of the Rydberg electron. Like the Born-Oppenheimer separation, its inverse can also be formulated in an adiabatic or a diabatic basis. The diabatic inverse Born-Oppenheimer is practical both for interpretation of zero electron kinetic energy (ZEKE) spectra and for computations. Explicit results are given for a model system of an electron orbiting a vibrating dipole, identifying the relevant coupling constants. The discussion emphasizes the radial motion and the limits discussed here are not quite equivalent to the four (or, actually, five) Hund's coupling cases relevant to angular momentum coupling schemes.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 85-100, 1998
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  • 18
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore the viability of a time-independent quantum adiabatic switching algorithm in the Fourier grid Hamiltonian (FGH) framework in the presence of degeneracy, avoided crossing, and chaos. The algorithm is simple and cost effective and provides information about the full eigenspectrum of the evolving Hamiltonian. It is shown to be capable of capturing accurately the change in the pattern of level spacing distribution statistics as one switches from a nonchaotic region of parameter values into the chaotic region. The Transition turns out to be less sharp than anticipated.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 133-141, 1998
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  • 19
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two points about correlation potentials have been dealt with in this article. The first one is related to the shape of some of the most representative correlation potentials applied to the ground state of the He atom. It is shown here that both LDA and two-body density correlation potentials compare well with that obtained through the quantum chemistry definition of correlation energy. This is an interesting result because, in previous works, it had been shown that none of the correlation potentials compared well with the Kohn-Sham one. The gradient-corrected correlation potentials exhibit a very different behavior to that of both exact potentials (quantum chemistry and Kohn-Sham ones). The other question posed here refers to how a reference to the two-body density must modify DFT functionals for the correlation energy, when a multideterminant wave function is needed. This question has been addressed by analyzing the variation of correlation potentials as the bond length of the H2 molecule increases. The results show that an external reference to the two-body density qualitatively improves DFT correlation potentials and also that only those functionals explicitly depending on two-body density can give the quantitative correct trends.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 143-156, 1998
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  • 20
    ISSN: 0020-7608
    Keywords: SN2 reaction ; reaction path Hamiltonian ; intrinsic reaction coordinate ; reaction path curvature ; ab initio calculation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To obtain essential information on the reaction dynamics for the prototype gas-phase SN2 reaction Cl-+CH3Cl→ClCH3+Cl-, the characteristic features of the potential energy surface in the local region around the reaction path were examined by the reaction path Hamiltonian constructed with high-level ab initio molecular-orbital calculations. After the structures of relevant stationary states and the intrinsic reaction coordinate were determined, the transverse vibrational modes, the corresponding frequencies, and the coupling elements between the pairs of normal modes induced by the reaction coordinate motion were calculated at each point along the intrinsic reaction coordinate. It was found that a quite large reaction path curvature appears in the intrinsic barrier slope near the bottom of each of the pre- and postreaction stable-state complexes. This large curvature was clarified to cause the internal vibrational excitation of the products and the requirement of the vibrational excitation of the reactants for reaction occurrence. The complex recrossings across the transition-state theory dividing surface, previously characterized by Hase et al. [J. Chem. Phys. 96, 8275 (1992)] in which trajectories trapped in the Cl-(DOTTED BOND)CH3Cl complex return to the central barrier region, were demonstrated to be attributed to this large curvature. Furthermore, not only the variational effects but also the reaction path curvature effects on the intermediate recrossings that were also characterized by Hase et al., in which trajectories linger near the central barrier, were found to be negligible.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 261-271, 1998
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  • 21
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: tight-binding ; band spectrum ; orbital nonorthogonality ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A one-dimensional one-band model is presented which enables a tight-binding calculation to be performed with full rigor. All the multicenter integrals involved in the calculation of the Hamiltonian matrix elements can be analytically computed in terms of a single parameter: the ratio between the lattice spacing and the extension of the “atomic” wave function. Moreover, the exact expression for the spectrum shows how the usual approximations of limiting the range of interactions and neglecting the nonorthogonality of the basis set degrade the results of the calculations.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 285-291, 1998
    Additional Material: 4 Ill.
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  • 22
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Boys function ; molecular integrals over Gaussian functions ; computation of Boys function ; error function erf(x) ; electron repulsion integrals ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose a new expansion for the Boys function ∫01t2jexp(-r2t2) dt appearing in the calculation of molecular two-electron matrix elements if Gaussian basis sets are employed. This expansion involves a power series involving the terms Ci, j(τ) (r2-R2)i multiplied by exp(-τr2), where τ is an optimized parameter τ∊[0, 1]. The performances of the introduced expansion are discussed and illustrated by some numerical experiments. It appears that the proposed expansion is considerably shorter than the customary Taylor series, which in turn is the special case for τ=0. This is of some importance, particularly for higher j values. Further, the proposed expansion enables a single expression for calculating erf(x) for the whole range of variable x. The recursive relations for the expansion coefficients are derived and the truncation errors are estimated. A new method for calculating the Boys function by means of asymptotic series is represented too.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 305-315, 1998
    Additional Material: 5 Tab.
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  • 23
    ISSN: 0020-7608
    Keywords: multiconfigurational SCF approach ; electron correlation ; high-symmetric molecules ; fullerene ; group theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A symmetry-adapted multiconfiguration self-consistent field (MC SCF) approach aimed at calculations of high-symmetry molecules is proposed. The self-consistency procedure applicable to the molecular terms of any symmetry and multiplicity is developed. It holds the symmetry transformation properties of varied molecular orbitals, thus taking advantage of the relationships within the set of two-electron integrals through molecular invariants. For orbital optimization, a unified coupling operator is constructed on the basis of the pseudosecular method providing for efficient convergence to energy minimum. Based on the group-theory technique, computer codes have been developed for straightforward determination of the invariant expansions for two-electron integrals and configuration interaction (CI) matrix elements. Calculated in this way, the expansion coefficients are presented for the three-electron states that originate from joint t1u and t1g shells of an icosahedral fullerene C60, the case important for the calculations of anion C603- representing the charge state of the fullerene molecule in the superconducting ionic solids K3C60 or Rb3C60. The results of MC SCF calculations for lowest quasi-π-electronic states of C603- are discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 293-304, 1998
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  • 24
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: INDO/CI calculations were used to analyze the C1s and O1s shake-up spectra of nickel tetracarbonyl, Ni(CO)4. The satellite structure in both cases is dominated by excitations from metal-ligand bonding (2Πb) to metal-ligand antibonding (2Πa) orbitals and by excitations within the core-ionized CO molecule, ΠCO - Π*CO.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 649-657, 1998
    Additional Material: 8 Ill.
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  • 25
    ISSN: 0020-7608
    Keywords: conjugated hydrocarbon ; internal rotation ; ab initio method ; geometry ; energy barrier ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The potential-energy curves of internal rotation were calculated for 1,3-butadiene at the MP2/6-311G** level, for isoprene and 1,3-pentadiene at the MP2/6-311G* level, and for 2,3-dimethyl-1,3-butadiene and styrene at the MP2/6-31G* level. The geometries of the energy minima (stable conformers) and maxima (transition states) on the curves are completely optimized. For butadiene and its methyl derivatives, two stable rotamers, s-trans and gauche conformers, are obtained. s-trans forms have the lowest energies and gauche conformers twisted by 39.9°-48.3° around the central bond of the butadiene skeleton are, on average, 9.8 kJ/mol above the trans forms. s-cis forms are rotational transition states. The computed gauche-cis barriers range from 4.30 to 11.70 kJ/mol. The regular effects of methyl substitutions at the end and central carbons are found. For styrene, the planar form is calculated to be a saddle point which is only about 1 kJ/mol higher in total energy than a twisted minimum, in which the torsional angle between the phenyl and vinyl planes is 27.4°.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 659-667, 1998
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  • 26
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 27
    ISSN: 0020-7608
    Keywords: ab initio calculations ; metal oxides ; oxygen anions ; methane ; surface reactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The stability of oxygen anions and the hydrogen abstraction from methane on a reduced SnO2 (110) crystal surface have been studied theoretically using a point-charge model. The geometric and electronic structures for the present molecules are calculated by means of a hybrid Hartree-Fock/density functional method at the B3LYP/6-311+G(3df, 3pd) level of theory. The calculations of the energies on the point-charge model are performed using these optimized geometries. It is found that a low concentration of the active oxygen species O- and O2- is expected on the reduced SnO2 surface. The activation energies for the abstraction of hydrogen atom from methane on the reduced SnO2 surface are obtained: 12 kcal/mol for O- species and more than 48 kcal/mol for O2- species, indicating that O- species on the surface is the main active center for the dissociation of a C(SINGLE BOND)H bond of methane, which is in agreement with the other oxide catalysts.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 669-678, 1998
    Additional Material: 6 Ill.
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  • 28
    ISSN: 0020-7608
    Keywords: aromaticity ; hydrogen clusters ; complete basis set ; Gaussian-2 ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Energies for highly symmetric cyclic hydrogen clusters (Hn) with Dnh symmetry were evaluated with Gaussian-2 (G2) and quadratic complete basis set (CBSQ) ab initio approaches. The energies were compared with linear hydrogen clusters as well as with their energies of formation from hydrogen and charged hydrogens. To better estimate aromaticity and eliminate ring strain energy, energies of formation for hydrogen clusters of the same ring size but with different electron numbers were compared. In this way, it was confirmed that the planar hydrogen clusters in singlet electronic configurations follow the Hückel aromatic rule, while aromaticity of the planar hydrogen clusters in an excited state (triplet) follows the Möbius 4n aromatic rule. It was also determined that hexagonal H6 is actually the transition-state structure for hydrogen exchange. The activation barrier for this exchange was estimated to be around 75 kcal/mol.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 679-687, 1998
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  • 29
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 30
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: BCS ; GL ; Hubbard-Stratonovitch ; Nambu ; superconductivity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is intended to derive the Ginzburg-Landau (GL) equation directly from the Bardeen-Cooper-Schrieffer (BCS) Hamiltonian. By the use of the Hubbard-Stratonovitch transformation, the electron-electron interaction composed of four fermion operators is eliminated to yield an auxiliary boson field. This is an effective field in which electrons behave as if they were free. In applying the path integral method, the electron field is integrated out to remain the Lagrangian for this boson field. The symmetry breaking and the phase transition of the system described by this field are discussed, and it is shown that this boson field turns out to be the GL order parameter.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 693-703, 1998
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  • 31
    ISSN: 0020-7608
    Keywords: infrared spectrum ; intensity ; frequency ; molecular mechanics ; quantum chemistry ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Infrared spectra simulations require ab initio techniques to get reliable intensities. On the other hand, recent force fields can provide accurate molecular geometries and frequencies. Therefore, it is suggested that these new force fields could be used to simulate infrared spectra, dipole-moment surfaces being described at high levels of theory in order to get satisfactory intensities. As pointed out, for a system with N atoms, the cost of such a simulation would be reduced N-fold with respect to standard quantum approaches. Preliminary calculations based on this scheme are reported here. Encouraging results are obtained since no significant lost of accuracy is noted on going from the ab initio to the molecular mechanics potential energy surface.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 705-711, 1998
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  • 32
    ISSN: 0020-7608
    Keywords: correlation energy ; MBPT ; level shift ; dissociation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effect of replacing the Hartree-Fock one-particle energies with ionization potentials obtained from inverse Dyson equation when calculating electron correlation energies perturbatively is investigated. Though the energy shifts vary from system to system, the slight decrease of the resulting excitation energies at around equilibrium geometries leads to a slight increase of the correlation energies in most cases. In the dissociation limit the inverse Dyson equation opens the gap, thus nondiverging potential curves emerge even at the restricted Hartree-Fock (RHF)+RS2 level.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 713-719, 1998
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  • 33
    ISSN: 0020-7608
    Keywords: molecular dynamics ; hydrogen bonding ; conformational studies ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thiamin diphosphate (ThDP) is an essential cofactor for a number of enzymes, especially of pyruvate decarboxylase (PDC) which catalyzes the decarboxylation of α-keto acids. Recently, the crystal structure of PDC-bound ThDP has been determined. Based on these X-ray data molecular dynamics (MD) simulations of the isolated coenzyme as well as of ThDP in its enzymatic environment were performed, using the GROMOS87 software package. In the ThDP-apoenzyme model all significant amino acid residues with a cut-off radius less than 8.5 Å from the cofactor were considered. The conformational behavior and the formation of specific structures of both ThDP and enzyme-bound ThDP were investigated in order to get hints on the activity and the mechanism of the coenzyme. Therefore, trajectories of significant structural parameters were analyzed by our graphics tool. Moreover, Ramachandran-like plots with respect to significant torsion angles were used for the illustration. Finally, MD simulations on ThDP analogs with less or none catalytic activity and apoenzyme mutants were included, in order to study the cofactor-apoenzyme binding.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 407-413, 1998
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  • 34
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A detailed study of the type and extent of electronic reorganization created by macroscopic solvation of a prototypical carbonyl solute is carried out within the framework of the semiempirical multiconfiguration self-consistent reaction-field model. The solvation causes additional polarization of the electronic charge density within the carbonyl group. Orbitalwise breakup of the electron densities on the key atoms throws light on such features as the polarity dependence of inversion barriers and out-of-plane bending. Macroscopic solvation is shown to cause a reverse π-polarization within the carbonyl group, affecting barrier heights on the inversion path.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 415-428, 1998
    Additional Material: 10 Ill.
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  • 35
    ISSN: 0020-7608
    Keywords: intramolecular H bonds ; gas-phase stability ; continuum solvent ; solvation free energy ; point charge model ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational properties of bilirubin present a considerable interest from the medical and biological points of view, because of the toxic pigment peculiar solubility features. Therefore, ab initio geometry optimizations at the 3-21G/SCF level were carried out on a few model-built structures of bilirubin, after substituting the methyl groups on the heteroaromatic rings with H atoms. The structures obtained show a varied network of H bonds, ranging from 0 to 6 H bonds for the lowest-energy conformer in vacuo. This conformer assumes a “ridge-tile” shape previously found in both MM/MD simulations [D. A. Lightner et al., J. Am. Chem. Soc. 114, 10123 (1992)] and semiempirical calculations [W. H. Shelver et al., Int. J. Quantum Chem. 44, 141 (1992)], but at the ab initio level, its computed H-bond geometrical parameters are fairly consistent with the X-ray observed ones. The relative stability of all the structures is compared in vacuo and in aqueous solution using the polarizable continuum model (PCM) of the solvent, whose stabilizing effect, however, is not sufficient to fill the large energy gap between the less lipophilic conformers and the most stable one in vacuo, in agreement with previous CD measurements. The energetic cost imposed to the solvent effect by each intramolecular H bond is almost constant if the other structural features do not change appreciably. The trend of the purely electrostatic results obtained employing a partial charge model of bilirubin is analogous to that produced by its polarized ab initio charge distribution.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 395-405, 1998
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  • 36
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: symmetric group ; class-sums ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An algorithm for the evaluation of products of arbitrary conjugacy class-sums in the symmetric group is conjectured. This algorithm generalizes a procedure presented sometime ago, which deals with products in which at least one of the class-sums involved consists of a single cycle (and an appropriate number of fixed points). Let the support size of a conjugacy class be the number of indices that are not fixed points. The algorithm proposed implies that the coefficient of the class-sum C in the product of the class-sums A and B is given in terms of a well-defined enumeration problem within the symmetric group Sp, where p is the smallest of the support sizes of A, B, and C.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 429-440, 1998
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  • 37
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: quantum fluid dynamics ; atom-laser interaction ; femtosecond dynamics ; density functional theory ; high harmonic generation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A comprehensive, nonperturbative, time-dependent quantum mechanical (TDQM) approach is proposed for studying the dynamics of a helium atom under an intense, ultrashort (femtoseconds) laser pulse. The method combines quantum fluid dynamics (QFD) and density functional theory. It solves a single generalized nonlinear Schrödinger equation of motion (EOM), involving time and three space variables, which is obtained from two QFD equations, namely, a continuity equation and an Euler-type equation. A highly accurate finite difference scheme along with a stability analysis is presented for numerically solving the EOM. Starting from the ground-state Hartree-Fock density for He at t=0, the EOM yields the time-dependent (TD) electron density, effective potential surface, difference density, difference effective potential, ground-state probability, 〈r〉, magnetic susceptibility, polarizability, flux, etc. By a Fourier transformation of the TD dipole moment along the linearly polarized-field direction, the power and rate spectra for photoemission are calculated. Eleven mechanistic routes for photoemission are identified, which include high harmonic generation as well as many other spectral transitions involving ionized, singly excited, doubly excited (autoionizing), and continuum He states, based on the evolution of the system up to a particular time. Intimate connections between photoionization and photoemission are clearly observed through computer visualizations. Apart from being consistent with current experimental and theoretical results, the present results offer certain predictions on spectral transitions which are open to experimental verification.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 441-474, 1998
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  • 38
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: NO2 ; DVR representation ; conical intersection ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using available potential energy surfaces, a theoretical calculation of the $\tilde{A}^2B_2\leftarrow \tilde{X}^2A_1$ absorption spectrum of NO2 is presented. Energy levels and nuclear wave functions are calculated, using a discrete variable representation (DVR) basis expansion to solve the nuclear Schrödinger equation. Various energy regions of the absorption spectrum are considered. In the low-energy region, below the onset of the $\tilde{A}^2B_2$ excited state, accurate results are recovered in a one-electronic-state calculation. It is also shown that using a decoupled potential energy surface derived from Schryber potential, accurate results are also obtained in the two-coupled-state case. Calculations are also performed in the higher energy part of the spectrum up to 16,000 cm-1. Analysis of the calculated wave functions makes the assignment of vibrational quantum numbers possible for both electronic components to the various excited levels. Comparison with experimental results are presented. Reasonable agreement is obtained for the transition frequencies. An overall reliable reproduction of experimental data, including the intensities through Franck-Condon calculation, requires, however, the transition moment function as additional information. General conclusions concerning the limitation of the model are discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 475-489, 1998
    Additional Material: 6 Ill.
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  • 39
    ISSN: 0020-7608
    Keywords: positron-molecule complex ; positron affinity ; full variational molecular orbital method ; nuclear wave function ; orbital relaxation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optimal Gaussian-type orbital (GTO) basis sets of positron and electron in positron-molecule complexes are proposed by using the full variational treatment of molecular orbital (FVMO) method. The analytical expression for the energy gradient with respect to parameters of positronic and electronic GTO such as the orbital exponents, the orbital centers, and the linear combination of atomic orbital (LCAO) coefficients, is derived. Wave functions obtained by the FVMO method include the effect of electronic or positronic orbital relaxation explicitly and satisfy the virial and Hellmann-Feynman theorems completely. We have demonstrated the optimization of each orbital exponent in various positron-atomic and anion systems, and estimated the positron affinity (PA) as the difference between their energies. Our PA obtained with small basis set is in good agreement with the numerical Hartree-Fock result. We have calculated the OH- and [OH-; e+] species as the positron-molecular system by the FVMO method. This result shows that the positronic basis set not only becomes more diffuse but also moves toward the oxygen atom. Moreover, we have applied this method to determine both the nuclear and electronic wave functions of LiH and LiD molecules simultaneously, and obtained the isotopic effect directly.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 491-501, 1998
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  • 40
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 41
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1099-1099 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 42
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: ring currents ; conjugated molecules ; Pariser-Parr-Pople model ; valence bond method ; correction vector technique ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have studied magnetism and aromaticity of polycyclic ring systems by analyzing ring currents for different circulations in these molecules. The technique employed for calculating ring currents uses correction vectors which implicitly includes all the eigenstates of the Hamiltonian in the space of the chosen configurations. We have employed the Pariser-Parr-Pople Hamiltonian and have carried out full configuration interaction (CI) calculations for small systems and approximate CI calculations for large systems. The systems studied include polyacenes, nonaromatic ring systems including the C60 fragments pyracylene, fluoranthene, and corannulene, and heteroatomic systems with upto two six-membered rings. We find that in polyacenes, the aromaticity of the extreme phenyl rings reduces with increasing number of phenyl rings in the system, and it saturates at ≈⅔ the benzene value. In systems containing nonaromatic rings, we find paramagnetic or diamagnetic behavior for different circulations depending upon the number of atoms in the chosen ring cycle, in agreement with the 4n+2 rule. In corannulene, the largest fragment of C60 we have studied, the five-membered ring is weakly diamagnetic while the six-membered ring is more diamagnetic, although much less than in isolated benzene. The ring structures with heteroatoms studied are pyridine, pyrimidine, and its isomers, s-triazine, quinoline and its isomer, and quinazoline and its isomers. All these have similar ring currents as in their purely carbon counterparts, although ions of these molecules show interesting behavior.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 503-513, 1998
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  • 43
    ISSN: 0020-7608
    Keywords: acetylcholinesterase ; acylation process ; transition state ; quantum chemistry ; catalytic mechanism ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The acylation process in the acetylcholinesterase (AChE)-catalyzed hydrolysis of the neurotransmitter, acetylcholine (ACh), has been determined with the semiempirical quantum chemical calculation method AM1 using the model molecules extracted from the X-ray crystal structure of Torpedo Californica AChE. For the sake of identifying the microfeatures of the mechanism of this reaction, two types of possible mechanisms, stepwise mechanism and cooperative mechanisms, were proposed and studied with AM1 methods. All the model molecules for the possible reactants, intermediates, transition states, and products in the reaction pathways of the two mechanisms were obtained. Energy profiles, the structural properties of the transition states, indicate that the acylation of AChE-catalyzed hydrolysis of ACh adopts the cooperative mechanism, i.e., the proton transfer from Ser220 of AChE to His440 occurs simultaneously with the nucleophilic attack of Ser200 to the carbonyl carbon atom of ACh. This result is in agreement with the kinetic data and the secondary isotope effects of AChE-catalyzed reactions.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 515-525, 1998
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  • 44
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The computed molecular surface electrostatic potentials of a group of anticonvulsants of various chemical types were investigated with the objective of identifying common features that may be related to their activities. The calculations were carried out with the density functional B3P86/6-31G* procedure, using HF/STO-3G*-optimized geometries. Analysis of several statistically based properties of the surface potentials indicates that the negative regions are of primary importance and that an optimum intermediate level of local polarity, or internal charge separation, is required.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1137-1143, 1998
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  • 45
    ISSN: 0020-7608
    Keywords: pharmacophoric pattern ; antiepileptic activity ; conformational analysis ; N-valproyl glycine ; N-valproyl glycinamide ; valpromide ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two closely related N-substituted valpromide derivatives: N-valproyl glycinamide and N-valproyl glycine are comparatively analyzed, the first of which is antiepileptic active whereas the second is not. The study is based on a conformational analysis using an AM1 Hamiltonian that not only search for the lower energy structures of each derivative but also for the energy involved in their mutual interconversion. Open structures have been compared with cyclic ones, the latter including those stabilized by either inter or intra molecular hydrogen bonds (dimers and monomers, respectively). H-bond formation has been also evaluated by means of ab initio G94(6-31+G(d,p)) calculations for a smaller system (N-formylglycine/glycinamide) modeling both vacuum and solvent conditions. The conformational and electronic characteristics of the open and cyclic monomers, as well as of the dimer N-valproyl glycinamide and N-valproyl glycine structures are discussed. On the basis of the results of their comparative analysis, we have redefined the pharmacophore previously proposed for N-substituted valpromides [Tasso, Bruno-Blanch, Estiu, Int. J. Quant. Chem. 65(6), 1107 (1997)], relaxing some of the associated requirements. The corrected model requires one carbon atom or any bioisosteric substituent in an anticlinal conformation relative to the aminic nitrogen of the amide moiety, in addition to one hydrogen atom that should be antiperiplanar to the carbonyl oxygen. This model offers an explanation to the different response of N-valproyl glycinamide and N-valproyl glycine against convulsion, which is based on conformational restrictions.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1127-1136, 1998
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  • 46
    ISSN: 0020-7608
    Keywords: nitrogenase ; nitrogen fixation ; INDO ; DFT ; PM3tm ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We follow the initial activation of the nitrogen molecule at the FeMo cofactor of nitrogenase and subsequently model the hydrogenation of N2 up to the fourth protonation step using the intermediate neglect of differential overlap quantum-chemical model. The results obtained favor a reaction mechanism going through hydrazido intermediates on the 4-Fe surfaces, externally to the FeMo cofactor. Calculations using density functional theory on smaller model systems also support the suggested mechanism over other possible schemes that involve early release of the first molecule of ammonia as a product of the enzymatic reaction. We also demonstrate that dielectric stabilization due to the protein around the cofactor could lower markedly the barrier for the product release as an ammonium ion.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1159-1168, 1998
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  • 47
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: RHF/6-31G* investigations of 4-, 5-, and 6-ethyl(Et)-indole-3-acetic acid (IAA) yielded 11 symmetry-unique local minima with syn-periplanar orientation of the —COOH group for each of these compounds. The global minima are of C1 symmetry in all cases. Comparison with earlier results shows that ethylation or chlorination in position 5 or 6 introduces only minor changes on the orientation of the acetic acid side group, with no effect on the reaction paths related to this group. For 4-Et-IAA, the deviations from unsubstituted IAA are larger but preserve the pattern of reaction paths that is present in unsubstituted IAA, which is in contrast to 4-Cl-IAA, where local minima and reaction paths are completely different.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1169-1175, 1998
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  • 48
    ISSN: 0020-7608
    Keywords: phytochrome ; absorption spectra ; semiempirical ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Higher plants use the protein phytochrome as a photosensor. In physiological temperatures phytochrome exists in two forms: Pr and Pfr. The chromophore of phytochrome is an open-chain tetrapyrrole. On the pathway from Pr to Pfr four intermediates (Lumi-R, Meta-Ra, Meta-Rb, and Meta-Rc) can be distinguished, while only two (Lumi-F and Meta-F) can be seen on the way back from Pfr to Pr. We have used the x-ray structure of the C-Phycocyanin protein Fremyella diplosiphon bacteria as a template to build a model (∼200 atoms) that includes only the chromophore and five amino acids of the phytochrome (Arg316-Cys321-His322-Leu323-Gln324) around it. Using the existing experimental evidences, we have proposed a three-dimensional (3D) structure for Pr, Pfr, and intermediates and a mechanism for the photoisomerization as well. Structures were fully optimized using AM1 (Unichem package on a Cray J90-NACAD). Using the INDO/S method of Zerner and co-workers, we calculated the absorption spectra of the model compounds and compared them with the experimental data. The oscillator strength ratio is an indicator of the chomophore conformation in biliproteins. The calculated spectra reproduces well the spectra of the phytochrome (Pr, Pfr, and intermediates) except for the lower energy band. This result is attributed to the small number of amino acids in the models. The calculated ratios (fVIS/fUV-fosc of visible band over fosc of UV band and f2/f1-fosc of second absorption band over fosc of first absorption band) for the models match very well the experimental ratios obtained for the phytochrome (Pr, Pfr, and intermediates). This supports the proposed mechanism for the photoisomerization process.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1145-1157, 1998
    Additional Material: 11 Ill.
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  • 49
    ISSN: 0020-7608
    Keywords: pKa shift ; acid dissociation constant ; triple helix ; dielectric constant ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energetics of formation of a triple-helical structure in homopurine-homopyrimidine mixtures has been modeled using Poisson-Boltzmann calculations. Oligomers with the sequence d(TC)n and d(AG)n form hydrogen-bonded triple-helical structures of the form d(TC)n⋅d(AG)n⋅d(TC+)n. The third base, a pyrimidine in this case, forms Hoogsteen-type hydrogen bonds with the purine, requiring that the cytosine residues of the third strand protonate at N3. The pKa of cytosine, 4.3 in the isolated solvated molecule, is raised by the strong electrostatic field in the triple helix. We have done calculations of the effective pKa of this cytosine and compared the results with experimental studies of triple-helix formation as a function of pH. This provides a test of various models of the dielectric constant for triplex DNA and its local environment.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1177-1184, 1998
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  • 50
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1185-1194 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The integrated molecular transform (FTm) is a unitary numerical index of structure that is capable of uniquely representing different molecular structure conformations with the exception of enantiomers. Other molecular indices have been derived from FTm as well as from the normalized molecular moment (Mn), for example, the analogous electronic and charge transforms (FTe and FTc) and moments (Me and Mc). In this study, each of these indices was calculated for up to 10 sampled conformations of each of the C1-C10 normal alkanes as they were subjected to a standard annealing process. Statistical analyses of the resulting data in the individual series and subsequent box plots, permitting facile examination of those results, indicated that the respective transform indices (FTm, FTe, FTc) are unique, that is, with no statistically significantly overlap across the series. For the Mn and Me indices, the numerical values for methane overlapped those of ethane in the first instance and both ethane and propane in the second. The Mc index values overlapped in several instances in the series. Inasmuch as the noted molecular indices are based only on parameters of structural origin, these results have profound implications for the correlation and estimation of properties derived not only from a general structure representation, but also for those properties which may be dependent on specific molecular conformations. This includes the potential for indices of molecular flexibility and conformationally dependent atomic electron densities.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1185-1194, 1998
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  • 51
    ISSN: 0020-7608
    Keywords: GABAB analogs ; pharmacophoric pattern ; molecular similarity ; quantum chemical calculations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In order to determine the structural requirements that are important for GABAB binding affinity, a quantum-chemical-based conformational study has been performed, followed by a similarity analysis which includes 12 GABAB analogs. Due to the flexibility of the structures, a semigrid GABAB analog [2RS-(5,5-dimethyl) morpholinyl-acetic acid] has been used as a template for the amonium moiety in order to help to identify the active conformation. Both in vacuo, and solvent-simulated calculations, for the physiological media modeled as water molecules, have been compared, for this analog, at ab initio (G94, 6-31+G(d,p)) and semiempirical (PM3) levels, respectively. On the basis of this comparison, the results of in vacuo PM3 calculations have been chosen for the similarity analysis. We have included, in the calculations, a group of molecules heterogeneous enough to become representative of the different families that can bind to the GABAB receptor site. Following their comparison we report the leading characteristics that can be related to their binding capability and define a pharmacophoric pattern for GABAB analogs. The latter is compared with the one previously found for the binding affinity at the GABAA receptor site.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1195-1208, 1998
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  • 52
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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  • 53
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1209-1215 
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on optimal molecular connectivity descriptors for nitrogen atoms in amines for use in structure-property correlations. The descriptors represent generalized molecular connectivity indices with adjusted diagonal entries in the adjacency matrices of the corresponding molecular graphs, such that the standard error in a regression for boiling points in a set of amines is minimized. Advantages of the so-optimized descriptors for multivariate regression analysis in structure-property-activity studies are discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1209-1215, 1998
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  • 54
    ISSN: 0020-7608
    Keywords: electron correlation ; reduced density matrices ; N-representability ; cumulants ; particle-hole duality ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Through the 3,5-contracted Schrödinger equation (3,5-CSchE) quantum energies and 3-particle reduced density matrices (3-RDMs) are determined directly without wave functions. Since the 3,5-CSchE involves the 5-RDM, its solution is indeterminate without N-representability conditions. However, the indeterminacy of the 3,5-CSchE may be removed through a reconstruction strategy for building the 4- and 5-RDMs from the 3-RDM. We present a systematic procedure for obtaining corrections for Valdemoro's reconstruction functionals from two complementary approaches, the particle-hole duality and the theory of cumulants. With the cumulants we are able to demonstrate that we have obtained all terms in the reconstruction functionals which may be written as antisymmetric products of the lower rdms. The cumulants allow us to understand the reconstruction functionals in terms of a renormalized many-body perturbation theory. The reconstruction functionals also lead to a natural generalization of Wick's theorem for evaluating expectation values of fermionic annihilation and creation operators with respect to correlated reference states. Previous work [Phys. Rev. A 57, 4219 (1998)] has explored the determination of correlation energy and 2-RDMs through the 2,4-CSchE, also known as the density equation. Because the reconstruction functionals employed with the 3,5-CSchE depend only on the antisymmetric products of lower RDMs in constrast to those used with the 2,4-CSchE, the 3,5-CSchE method presented here does not require the solution of systems of linear equations during reconstruction or the storage of the reconstructed RDMs. Application of the 3,5-CSchE technique to a quasi-spin model generates ground-state energies and 2-RDMs similar in accuracy to single-double configuration interaction (SDCI). We employ a simple iterative procedure for the solution of the 3,5-CSchE without traditional diagonalization. The CSchE techniques offer an approximate solution of the N-representability problem and a new approach to electron correlation.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 557-570, 1998
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  • 55
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Optimum geometries are computed at both the SCF level of theory and the level of second-order perturbation theory for several isomers on the potential energy hypersurfaces of GeCH2, GeSiH2, and Ge2H2, including linear structures, methylene-carbenelike structures, dibridged structures, and monobridged structures. In addition, harmonic vibrational frequencies are computed to characterize these structures as local minima or transition states. All computations employ basis sets of triple-zeta quality on valence electrons with d and f polarization functions on the heavy atoms and p functions on hydrogen. This investigation is the first systematic study to include all of these germanium systems and to employ f-type polarization functions in such a study. Previous investigations of ours indicate that large basis sets such as those employed in this study can, in part, compensate for the lack of a more advanced treatment of electron correlation. While a dibridged global minimum is confirmed for both Ge2H2 and GeSiH2 systems, the C2v isomer, methylenegermene, is found to be the most stable structure for GeCH2.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 925-932, 1998
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  • 56
    ISSN: 0020-7608
    Keywords: time-dependent density-functional theory ; excited state surfaces ; avoided crossings ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This work reports the first density-functional theory (DFT) treatment of excited-state potential energy surfaces exhibiting avoided crossings. Time-dependent DFT (TD-DFT) results, using a recently proposed asymptotically corrected local density approximation functional, are compared with multireference doubles configuration interaction (MRD-CI) results for the 1A1 manifold of the CO stretching curves of planar formaldehyde. TD-DFT is found to reproduce the qualitative features essential for understanding the spectroscopy of this manifold, specifically the strong mixing of the 1(π, π*) with Rydberg transitions and the resultant avoided crossings.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 933-941, 1998
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  • 57
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: If one is to calculate the stopping power for a specified projectile-target combination within the energy interval of applicability of modified Bethe-Bloch theory, values of several parameters appearing in the formulation must be ascertained. In the past, the author has established such values for numerous target materials through fits of stopping-power measurements with modified Bethe-Bloch theory. However, the semiconductor materials ZnSe and GaAs have not yet been thus characterized. A set of very recent measurements of the stopping powers of each compound for low-energy protons and alpha particles, reported by members of the Helsinki group, has been analyzed in order to remedy this dearth of parameter values. Moreover, some corresponding measurements for 7Li ions traversing ZnSe have been analyzed for the purpose of obtaining the value of a single effective charge parameter. Results of these studies are reasonably consistent with expectations, and values are recommended for the mean excitation energy and the Barkas-effect parameter for each compound.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 919-924, 1998
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  • 58
    ISSN: 0020-7608
    Keywords: electron scattering ; effective potential ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The differential scattering cross-section (DCS) for electrons scattered elastically by neon and argon atoms is studied using a model potential. In the present study the long-range polarization potential is represented by an energy-dependent function, and the short-range part is constructed from the nonrelativistic Hartree-Fock wave function of the target atom. The computed differential cross section obtained using the approximate effective interaction potential for electrons scattered by neon and argon atoms in their ground state is compared with available published results. In the present study the parameters contained in the energy-dependent effective potential are determined by the minimization of the DCS with respect to angle θ and the incident energy. The resulting DCS in the angular range 2°〈θ〈178° is found to be in good agreement with the available experimental and theoretical results in the intermediate energy range.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 943-950, 1998
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  • 59
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A density functional study was used to investigate potential energy surfaces of the oxidative addition reaction CpM(CO)+CH4→CpM(CO)(H)(CH3) (M=Ru-, Os-, Rh, Ir, Pd+, and Pt+). A qualitative model which is based on the theory of Pross and Shaik was applied to develop an explanation for the barrier heights. As a result, our theoretical findings suggest that the singlet-triplet splitting (ΔEst=Etriplet-Esinglet) of the CpM(CO) species can be a guide to predict its reaction activity and enthalpy for oxidative additions. A better linear correlation was found between the latter two quantities. Considering the nature of the metal center, the following conclusions therefore emerged: For the 16-electron CpM(CO) system, a heavier transition-metal center (i.e., the third-row) will lead to a smaller ΔEst and, in turn, will facilitate the oxidative addition reactions to alkane C—H bonds. In contrast, a lighter transition-metal center (i.e., the second row) will result in a larger ΔEst and then tend to undergo the reductive elimination reactions of the C—H bond-forming. The results obtained are in good agreement with the available experimental results and allow a number of predictions to be made.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 961-971, 1998
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  • 60
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We developed an empirical potential for interactions between Si and N to describe silicon nitride systems using the Tersoff functional form. With this model, we explored the structural properties of amorphous silicon nitride through the Monte Carlo simulations and compared them to available experimental data. The empirical model provided a very good description of such properties for a-SiNx (0〈x〈1.5). Electronic structure of amorphous and point defects in crystalline silicon nitride were then studied using first-principles calculations. For such calculations, the configurations were created by the empirical model, with the relaxed structures used as input for the first-principles calculations. Atomic relaxation was later allowed in the first-principles calculations.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 973-980, 1998
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  • 61
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structural, electronic, vibrational, and topologic properties of a series of acid sites of zeolites were studied at different levels of ab initio molecular orbital theory. The zeolite acid sites were modeled by using the following molecular clusters: silanol H3SiOH (B0) and the clusters H3SiO(H)AlH3 (B1), (OH)3SiO(H)Al(OH)3 (B1―OH), and H3SiO(H)Al(OH)2SiH3(B2). The calculation of geometries and properties of these clusters were performed at the Hartree-Fock level, and, additionally, second-order Møller-Plesset (MP2) and density functional BLYP calculations were carried out for silanol and B1 clusters. Geometries were fully optimized by following Cs symmetry restrictions. The standard STO/6-31+G(D, P) basis set, which includes polarization and diffuse functions, was used for all the calculations. The topologic properties of the zeolite acid clusters, based on the theory of atoms in molecules, were analyzed in terms of the total density and the Laplacian density properties, both evaluated at the position of the bond critical points. The calculations showed that the frequency of the OH vibrational modes of the zeolite acid sites, often used as an infrared index for characterizing the acidity of zeolites, is linearly related to the total density of the charge at the critical points of the OH bonds, with a correlation coefficient of r2=0.97. These results indicate that the total density of the electronic charge at the critical point of the OH bond can be used as a tool for interpreting the structural and electronic features of the zeolite hydroxyl groups. A relationship between the Mulliken population of the H atom of the OH bond and the OH frequency gives a correlation coefficient of 0.67. On the other hand, the values of the Laplacian density calculated at the critical points of the bonds of the acid sites indicate that the zeolite structure is dominated by a network of Si—O and Al—O ionic interactions, while the O(SINGLE BOND)H bonds are characterized as covalent bonds, with different extents of charge concentration.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 951-960, 1998
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  • 62
    ISSN: 0020-7608
    Keywords: Mulliken charges ; zeolite ; periodic Hartree-Fock ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Distributed multipole analysis (DMA) on the basis of periodic Hartree-Fock (PHF) calculations, using the CRYSTAL code, is applied to five different all-siliceous zeolite models: chabazite, gmelinite, merlinoite, montesommaite, and RHO. Mulliken charges of the framework atoms were calculated with a pseudopotential ps-21G* basis set for silicon and a 6-21G* basis for oxygen. The charge values of the silicon atoms were approximated by a simple one-dimensional function with respect to the average Si—O distance within the respective SiO4 tetrahedra, whereas a two-dimensional function with respect to the average Si—O distance and the Si—O—Si angle was used for the oxygen atoms. Both dependences were then utilized to evaluate the Mulliken atomic charges of 10 other frameworks with a larger number of atoms per unit cell. The validity of such application is confirmed by comparison with results obtained through direct PHF calculation for all-siliceous mordenite.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 993-1001, 1998
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  • 63
    ISSN: 0020-7608
    Keywords: molecular similarity ; electron density contours ; fused-sphere surfaces ; van der Waals radii ; hydrocarbons ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Continua of molecular surfaces have been proposed in the past as realistic approaches to modeling molecular shape. A continuum of fused-sphere surfaces combines the simplicity of computations involving hard spheres with a more accurate description of the “fuzzy” boundary of a molecule. In this work, we study some simple properties of a continuum of molecular surfaces derived by linearly scaling the van der Waals radii. We introduce some molecular shape descriptors derived from surface area and volume and study their dependence on a scaling parameter that “swells” the molecular surface. As one moves away from the nuclei, any transitions in molecular shape are reflected by changes in the descriptors. These descriptors convey essential shape features, in the sense that they are rather insensitive to molecular size. Even though the geometrical descriptors characterize the continuum globally, their behavior appears to be strongly determined by local molecular shape features. The procedure can also be extended to more realistic electron density surfaces. As a tool, the method can be helpful in assessing molecular similarity, as well as in studying properties of local neighborhoods within large clusters.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 981-992, 1998
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