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  • Atomic, Molecular and Optical Physics  (301)
  • 1990-1994  (301)
  • 1970-1974
  • 1965-1969
  • 1993  (301)
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  • 1990-1994  (301)
  • 1970-1974
  • 1965-1969
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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 2
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 3
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new inequality and an approximate relation of incomplete Gamma function En(x) are derived in this paper. Applying both to simplify the error analysis on the recursive algorithm of En(x), we analytically derived a criterion about relative error. The criterion decides whether the forward recursive relation can be used to compute {En(x), n = 0, 1, ⃛, N} with an acceptable loss of significant figures. Further analytical exploration leads to another criterion that decides whether the group of En(x) can be evaluated with an acceptable absolute error by the same methods. Both criteria are not only superior to those given in previous work by other authors, but also strongly supported by our numerical experiments. A strategy is illustrated to explain how to apply both criteria to evaluate molecular integral over Slater-type orbitals. © 1993 John Wiley & Sons, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Löwdin α-function method, in which displaced orbitals are expanded in an infinite series of spherical harmonics, is implemented for Slater-type orbitals using a commercial computer algebra program, Mathematica, The program, which is included, generates a C matrix with integer elements that characterizes our approach to multicenter molecular integrals. The general two-center, two-electron Coulomb repulsion integral is produced analytically with a finite number of terms. Each Coulomb formula may be evaluated to arbitrary precision, since Mathematica works with integer arithmetic. Hence, cancellation errors can be overcome. © 1993 John Wiley & Sons, Inc.
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  • 5
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The X-NO2 rotational energy barriers of nitromethane, nitroethylene, nitrobenzene, and a group of nitramines have been computed using a local density functional (LDF) procedure, using ab initio Hartree-Fock (HF)-optimized structures of the ground and rotational transition states. The results have been discussed in relation to HF and some correlated ab initio values and the available experimental data. Our LDF barriers are overall quite reasonable, in generally satisfactory agreement with the experimental and correlated ab initio results. © 1993 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a theoretical and numerical comparison between two DIIS algorithms to which we refer herein as C1-DIIS and C2-DIIS. The C2-DIIS algorithm is shown to be equivalent to the traditional C1-DIIS algorithm near convergence and more stable numerically as regards the accumulation of round-off error when the C1-DIIS equations become (nearly) singular. The C2-DIIS algorithm also offers alternative solutions to the DIIS equations when anharmonicities or numerical difficulties are problematic. These properties of the C2-DIIS algorithm allow the use of much larger DIIS spaces, obviating the need for truncation of the DIIS space, which, when done, usually results in a loss of information contained in the iterated subspace. The C2-DIIS method can be considered to be a variant of the singular value decomposition method. © 1993 John Wiley & Sons, Inc.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using the density-functional theory in the local-density approximation, norm-conserving pseudopotentials, and plane-wave expansions, we have developed a simple way to calculate ab initio the uniaxial strain dependence of the TO(Γ) phonon mode in GaAs, including the envolved relaxations effects. The relaxation response of the system was evaluated from the use of the elasticity theory. Our results confirm the experimental evidence and show a reasonable agreement with the observed data for the frequency splitting of this phonon mode. © 1993 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A variational principle for the transition matrix is considered. Conception of the MC SCF (in particular, CASSCF) approach for the transition matrix is proposed as a solution of the variational problem on the optimal evolution of a packet of two stationary states with some additional conditions. An ordinary MC SCF method for a single state is a special case of the proposed approach. Some aspects of the solving of the equations for the optimal transition matrix are treated. The method can be used in atomic and molecular calculations of transition energies, oscillator strengths, and other properties. © 1993 John Wiley & Sons, Inc.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This is the first application of a rigorous, established multiple time-step method to ab initio molecular dynamics. The resulting algorithm is conceptually simple and easy to implement, but very effective. It translates the large mass differences present in ab initio molecular dynamics into substantial savings in computer time while retaining high accuracy. This transforms ab initio molecular dynamics from a desirable but prohibitively expensive possibility into a viable method, at least for short-time phenomena in small systems or for otherwise inaccessibly complicated potential energy surfaces. © 1993 John Wiley & Sons, Inc.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio electronic structure calculations are reported for low-lying electronic states, 1A1, 1A2, 3A2, 1B1, 3B1, 1B2, and 3B2 of the FNO2 molecule. Geometric parameters for the ground state 1A1 are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hylleraas-CI calculations with linked correlation terms of the form rpijrqkl are discussed. Formulas for the integration of the angular part are deduced and a method for the reduction of the radial part to auxiliary integrals is given. In the case of the Li atom, it is shown that for the calculation of the ground-state energy an ansatz for the wave function with at most two factors rpijrqkl is sufficient to achieve spectroscopic accuracy. © 1993 John Wiley & Sons, Inc.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some operators considered conventionally as Hermitian are not strictly Hermitian for the freeelectron states known especially in the case of solids. The consequences of this fact on the closure property are examined. © 1993 John Wiley & Sons, Inc.
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  • 14
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydration to the aldehyde group in nitro-substituted benzaldehydes was studied theoretically by using an AM1 method. The calculated values of activation energy and heat of reaction showed good correlations with the experimental values of rate constants and equilibrium constants, respectively. To examine the hydration mechanism, localized molecular orbitals were calculated. Furthermore, expression of the molecular orbitals of supermolecular complexes in terms of occupied and vacant orbitals of the component molecules elucidated that the interaction between the lonepair orbital of water and the antibonding C—O π-orbital of aldehyde was important. 2,6-DNBAl was concluded to be less reactive toward water than was 2.4-DNBAl. This is consistent with the experimental result that the mutagenic activity of 2,4-DNBAl depends not only on the reduction of the nitro group by a bacterial enzyme but also on the acetyl conjugation, but that in the case of 2,6-DNBAl the mutagenic activity is manifested through the reduction by a bacterial enzyme. © 1993 John Wiley & Sons, Inc.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 16
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The spatial symmetries of the harmonic oscillator and the recently found irreducible tensors constructed from the associated annihilation and creation operators are exploited to obtain new expressions for the elements of the matrix representatives of several examples of vibrational operators. Since all vibrational operators are expressible in terms of the irreducible tensors, their matrix elements reflect the angular momentum symmetry inherent in them, for the results derived here are in terms of the Clebsch-Gordan coefficients and the isoscalar factors that arise from the couplinig rule of the irreducible tensors. Familiarity with the mathematical properties of these quantities derived from the elementary theory of angular momentum facilitates the evaluation of many vibrational operators that may be of importance in the study of potentials in this basis. In particular, it is shown that the nonvanishing of matrix elements is governed by a law of conservation of angular momentum along the axis of quantization of the nondegenerate harmonic oscillator. © 1993 John Wiley & Sons, Inc.
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  • 17
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A potential-derived atomic multipole method called the cumulative potential-derived atomic multipole method is developed, with which electrostatic atomic multipole moments are derived by fitting the molecular electric potential in a cumulative way. It is applied to the hydrides of N, O, F, S, Cl, and methanol and the hydrogen-bonded dimers formed between them. The relationship between atomic multipole moments and molecular charge distributions is found. The structures calculated with Buckingham's electrostatic model are in good agreement with experiments. The phenomena of nonlinear structures of most H-bonded complexes - the deviations of symmetry axes of electron donors from H bonds - and correct distinguishing between two alternative structures are attributed to atomic dipole and quadrupole moments. Compared with other methods, this method has a quantitative and qualitative advantage and simple algorithm. The main conclusion is that the atomic multipole moments play a substantial role, although a potential-derived charge model was deemed sufficient previously. © 1993 John Wiley & Sons, Inc.
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  • 18
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Structure and the energy of the stable conformations of the two possible stereochemical configurations of 2,4-diphenylpentane are obtained using the ab initio molecular orbital theory. The objective was to mimic the possible structures and determine the corresponding energies of the dyads of syndiotactic and isotactic polystyrene and, consequently, to study the interactions between the phenyl groups. The results of complete geometry optimization showed significant changes in geometrical parameters compared with those expected from the ideal hydrocarbon structure. The steric strain is most pronounced in some of the gauche conformations where large (approximately 40°) distortions of the backbone torsional angles and/or simultaneous phenyl group rotations in the range of 30°-40° away from its global minimum position may occur. In addition to the discussion of the geometrical parameters, the corresponding dipole moments are also calculated and differences related to the various structures and discussed. © 1993 John Wiley & Sons. Inc.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 20
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A brute-force numerical investigation has been carried out on the hopping of excitons in a three-dimensional molecular aggregate. Possibilities of vibronic decay, rapid chemical reactions of saturated species, radiative decay of overpopulated molecules, and cooperative chemical reactions involving saturated exciton populations on traps of two different types have been considered. Investigation have been performed with two types of initial distribution of excitons - facial and random - and for 10,000 or, sometimes, for 20,000 time steps each of duration 1ps. Several interesting observations have been made from this computer experiment: (1) The total number of occurrences of fast reactions depends upon the initial distribution of excitons. (2) It decreases if other exciton depleting processes are at work. (3) It also depends on the pattern of placement of traps. (4) The location of impurities also affects the rate of occurrence of these reactions. Thus, more reactions occur when the excitons are initially concentrated on one face and traps are suitably located on the path of flow of these excitons. A random initial distribution tends to equilibrate the excitons quickly over all the lattice points, thus giving rise to fewer reactions. (5) The number of reactions need not necessarily increase with the number of reaction centers; in fact, it decreases as more centers are added when the supply of excitons is severely limited. (6) A Complicated dynamics results when different types of additional processes, viz., enhanced fluorescence, radiative emissions, and cooperative chemical reactions are simultaneously allowed. The cooperative process has been clearly found to dominate. A first-order rate constant of about 108 s-1 has been calculated for the occurrence of the cooperative process. This rate is affected when other nonconserving processes are switched on. Observations (1), (4), and (5) are the most important conclusions of our work. They lie outside the scope of traditional models such as the random walk model, the diffusion model, and the lattice model for the migration of excitons in a molecular aggregate. © 1993 John Wiley & Sons, Inc.
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  • 21
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An implementation of the complex coordinate method is demonstrated that exploits a new technique for obtaining the matrix representation of the complex dilated Hamiltonian. The purpose is to make the complex coordinate method applicable together with standard numerical ab initio codes designed for large-scale calculations on many-electron atoms and/or molecules. No complex intergrals have to be calculated, and no changes of the standard codes are required even in the common case where the kinetic and potential energy components are not stored separately. Instead, two standard (real) CI calculations are used to generate the dilated (complex) CI matrix representation. The performance of the procedure is demonstrated in the context of the GAMESS program and applied to obtain the resonant structure of the Bethe surface pertinent to the absorption spectrum of the helium atom. © 1993 John Wiley & Sons, Inc.
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  • 22
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Complex Coordinate Scattering Theory is reformulated for the general case of a time-independent Hamiltonian. It is applied to scattering of He atoms from a Cu(115) crystal surface by constracting the Green operator for the T-matrix from the eigenvectors of both the complex scaled Hamiltonian and its transposed (“right” and “left” eigenvectors), which are different in this case. The weakly asymmetric corrugation function describing the (115) face of Cu is shown to cause a strong dependence of the calculated diffraction intensities upon the direction of the incident atomic beam. The calculated transition probabilities are in excellent agreement with the experimentally measured ones, previously obtained by Perreau and Lapujoulade. We show that additional information about the gas atom/surface physisorption interaction potential can be obtained if the incident angle of the atomic beam (the angles between the beam and the surface normal) is changed from γ to -γ. © 1993 John Wiley & Sons, Inc.
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  • 23
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three hierarchically ordered topological factors, i.e., atom connectivity, centrality, and cyclicity, were found to control the number of self-returning walks (SRWS) associated with every atom in the molecule. The reversal of their order was observed in a few cases where the central location of atoms had a stronger influence than did their connectivity on the number of SRWS. Three atomic topological indices, i.e., the Morgan extended connectivity, the Balaban, Mekenyan, and Bonchev hierarchical extended connectivity, the Randić atomic path code, were found to closely match the ordering of atoms in molecules determined by their number of SRWS. New atomic graph invariants fi = lim SRWin/SRWn and ti = fi · SRW2 were specified and may find application in QSAR and QSPR. The ti indices are nonintegers close to atomic valence. The fi indices represent the limit of the number of SRWS of length n for the atom i, SRWin, normalized by dividing it by the total number of SRWS for the molecule. In the case of Hückel MO considerations, these invariants were shown to be numerically equal to the partial electronic charges of the lowest occupied molecular orbital (LOMO). A new class of isocodal atoms (atoms having the same number of SRWns) was observed, i.e., atoms that become isocodal only at n ≥ 1. A number of open questions following from these findings were formulated, including the possibility for a topological modeling of electron correlation. © 1993 John Wiley & Sons, Inc.
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  • 24
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 26
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Accurate electron number densities and interelectronic distance (intracule) densities are given in simple algebraic form for the ground states of H-, He, Li+, Be2+, B3+, and Ne8+. The densities are obtained from systematic sequences of increasingly accurate Hylleraas-type wave functions. The most accurate densities are obtained from 204-term wave functions that lead to variational energies no more than 16 nano-Hartrees above the exact the exact ones. The algebraic permit the evaluation of systematic sequences of many expectation values. Some moments of the densities are evaluated in this manner. © 1993 John Wiley & Sons, Inc.
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  • 27
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that the many-electron wave function can be expanded optimally in terms of a set of slater determinants that reduces considerbly the number of determinats differing by a single-particle wave function. The method is based on a variational procedure. The first step of the present method is the Brueckner optimization. The other determinants of the expansion are obtained by successive optimizations performed under additional constraints. The method is illustrated by specifically three-and four particle example. Finally, it is concluded that the eigenstates of a many- particle system can be approximated by states that neglect single-particle excitations. The Zero-order approximation of the present scheme is the Hartree-Fock approximation. © 1993 John Wiley & Sons, Inc.
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  • 28
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 29
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Multiconfigurational second-order perturbation theory is tested for the calculation of molecular structure and binding energies. The scheme is based on the Complete Active Space (CAS) SCF method, which gives a proper description of the major features in the electronic structure, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second-order perturbation theory with the CASSCF wave function as the reference state (CASPT2). The approach is applied to the calculation of equilibrium geometry and atomization energies for 27 benchmark molecules containing first-row atoms (the “G1” test). Large atomic natural orbital (ANO)-type basis sets are applied (5s4p3d2f for Li—F and 3s2p1d for H). It is shown that the CASSCF/CASPT2 approach is able to predict the equilibrium geometry with an accuracy better than 0.01 Å for bond distances and 0°-2° for bond angles. Calculated atomization energies are underestimated with between 3 and 6 kcal/mol times the number of extra electron pairs formed. The error in the heat of reaction for a number of isogyric reaction (no difference in number of pairs) varies between -2.5 and +1.0 kcal/mol. The same type of accuracy is obtained in calculations for excited states. The molecules B2, C2, FO, FOO, and FOOF have also been studied. Results for the first three molecules are in accordance with those of the benchmark molecules. The FO bond distance in FOO is predicted to be 0.02 Å longer than experiment. The heat of formation for FOO is computed to be 2.9 kcal/mol with an uncertainty of ±3 kcal/mol. Preliminary results for FOOF (obtained with a smaller basis set) indicate that the approach yields a somewhat too long FO bond distance (1.64 Å compared to 1.58 Å experimentally). © 1993 John Wiley & Sons, Inc.
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  • 30
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a new numerical method for solving the Schrödinger equation in the case of scattering or tunneling states. As an example, we study a model of the scanning tunneling microscope. The method uses a finite-element approximation of the wave function in the region of the scattering potential. From the wave function that we obtain, we derive the tunnel current density. © 1993 John Wiley & Sons, Inc.
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  • 31
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A program to optimize the structure of large molecules at the Hartree-Fock level of theory running concurrently on a network of workstations is presented. Problems encountered in obtaining nearly optimal speedup and their solutions are discussed. A simple scheduling algorithm is presented that enables up to 99.5% of the code to run in parallel. © 1993 John Wiley & Sons, Inc.
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  • 32
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We used a macroscopic dielectric model to study the effects of solvation and interaction between titratable and permanent partial charges on the protein conformational energy and the acid-base equilibria in the cyanobacterial photoreceptor phycoerythrocyanin, whose photoreversible photochromic response is attributed to a Z/E isomerization of the covalently bound tetrapyrrole chromophore. The calculations revealed the stabilization of the charged protonation state of the chromophore by a small set of strong local interactions. Although the protein is globular and water-soluble, the complex counterion structure has a striking similarity to the arrangement found for the photochemical active transmembrane protein bacteriorhodopsin. This could be attributed to the fact that the protonation site in the α-subunit of phycoerythrocyanin is buried in the interior of the protein. Due to the strong shielding from solvent, the interaction pattern is conserved upon a ground-state isomerization of the chromophore. The partial solvent exposure of the isomerization site resulted in a drastic influence of the chromophore configuration on the aqueous solvation energy of the protein. Implications for the sensitivity of the photochemistry to environmental factors and molecular binding are discussed. © 1993 John Wiley & Sons, Inc.
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  • 33
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    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 34
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    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 35
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Metropolis Monte Carlo and Poisson-Boltzmann calculations were done to quantitatively assess the conditions under which counterion condensation (CC) theory could be considered valid. The fundamental prediction of condensation theory, that the number of counterions bound to a polyelectrolyte molecule can be predicted by a single parameter describing the linear charge density of the charged system, was shown to be quantitatively correct for a range of conditions. To define the number of counterions bound, it was necessary to use an energy-based criterion by which ions that interact with the polyion with an energy less than -kT were considered bound. Using this criterion, Monte Carlo calculations on systems consisting of charged cylinders and a neutralizing number of counterions in a dielectric continuum showed that the number of bound counterions was quantitatively predictable by the CC relation (1-1/ξ) for systems with a linear charge density and dimensions approaching those of duplex or triplex DNA. Poisson-Boltzmann (PB) calculations on cylinders with different linear charge densities and radii have been done to assess the limits of the CC prediction that the number of counterions bound is a constant even as the bulk concentration of electrolyte in the surrounding is increased. As in the case of the MC calculations, the validity of the CC prediction is seen to increase with increasing linear charge density of the charged cylinder. The agreement between PB and CC is seen to be very good for highly charged cylinders. The results described here provide justification for the use of CC theory for interpreting experimental data on polyelectrolytes of the approximate dimensions and linear charge density of duplex or triplex DNA. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 37
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 38
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Compact expressions are presented for the 3n-j symbols, where 1 ≤ n ≤ 4, which feature sums over products of binomial coefficients, and certain integer triangular coefficients. The triangular coefficients in turn can be expressed as products of binomial coefficients. Thus in the formulas presented for the 3n-j symbols, the dependence on numerous factorials, formally as well as computationally, has been completely eliminated. While formulas which incorporate summations over products of binomial coefficients have been known for the 3- j and 6- j symbols, the introduction of the triangular coefficients, and the application of the binomial/triangular scheme to 3n-j symbols with n 〉 2, provide important new results. The new formulas are simpler, and they permit more efficient computations of the 3n-j symbols, both in exact and in floating point format, than most schemes which are currently in use. © 1993 John Wiley & Sons, Inc.
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  • 39
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Statistical electron correlation coefficients provide overall measures of the difference between the electron pair density and the product of one-electron densities. Explicitly correlated wave functions are used to compute radial and angular correlation coefficients for 29 low-lying states (n1S, n3S, n1P, n3P, n1D, and n3D with n 〈 7) of the two-electron ions from helium through Ne8+. This data base of 1566 correlation coefficients permits a systematic study of trends with respect to variation of nuclear charge, degree of excitation, angular momentum, and spin multiplicity. © 1993 John Wiley & Sons, Inc.
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  • 40
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Supermolecular model of the intermolecular interaction energy is studied. The correlation energy components are calculated by using the many-body perturbation theory (MBPT) in the canonical representation as well as in the localized one. The components of the corrections of the different order are analyzed individually. The effects of the localization procedure and the basis set are also investigated. An H2O trimer is used to study the two- and three-body components of the interaction energy. © 1993 John Wiley & Sons, Inc.
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  • 41
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A review is presented of our recent work which deals with some aspects of the inclusion of connected triple excitations in single reference coupled-cluster theory. We first consider basic aspects of the theory and computational cost, noting the need for accurate and economical approximations to the complete coupled-cluster single, double and triple excitation method (CCSDT). Several such approximations are now available, including the CCSD + T(CCSD) and CCSD(T) methods. We have now efficiently implemented analytical derivative methods for these and related methods for open-shell reference determinants, including restricted open-shell Hartree-Fock for CCSD(T). Some results with large basis sets are presented for the HCO radical which further demonstrate the reliability of these methods for the spectroscopic characterization of small open-shell species. The remainder of this article deals with several applications of the complete CCSDT method, a new implementation of which we have recently completed. Like our previous work, this is for general single reference determinants, although we have now incorporated Abelian symmetry. We present a comparison with full configuration interaction data for CH3; a study of the N2 potential curve using restricted and unrestricted Hartree-Fock references; a comparison of the dipole moments and polarizabilities of BeO; and a study of the harmonic force field of 2Σu+ C3+. These studies also provide assessments of the accuracy of the approximate schemes for including T3. Finally, we present some timing data for some CCSDT calculations with out new implementation. © 1993 John Wiley & Sons, Inc.
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  • 42
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A serious problem in the ab-initio Hartree-Fock calculation for extended systems is a singularity in the exchange contribution to the band energies (the “exchange pathology”). In the homogeneous electron gas this pathology is exactly cancelled by the correlation contribution arising from the sum of all time-ordered ring diagrams (RPA approximation). In this article we show that this cancellation is more general and does also occur in periodically extended systems. © 1993 John Wiley & Sons, Inc.
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  • 43
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ground state electron correlation is introduced into the one-particle propagator via coupled cluster theory. This defines a similarity transformation of the Hamiltonian, which leads to the complete separation of the ionization and electron attachment aspects of the propagator. The latter makes it possible to solve for each property independently. Furthermore, the frequency (or energy) dependence which characterizes propagator theory is eliminated by introducing a wave operator formalism. It is shown that this procedure is equivalent to the summation of certain types of terms in the electron propagator perturbation expansion to infinite order. Finally, the resulting equations are found to be equivalent to those of the Fock-space coupled-cluster (or equivalently the equation of motion coupled-cluster) method, which provides an explicit wave function for each state, demonstrating the connection between these different approaches for the calculation of ionization potentials and electron affinities. Understanding this relationship permits new and powerful approximations to be proposed. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
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  • 45
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article we calculate the asymptotic structure of the exchange potential for the Fermi level electron in the vacuum region outside a metal surface, and show it to be the image potential. In this asymptotic region the exchange potential due to the Fermi level electron is equivalent to the optimized potential of exchange-only density-functional theory. Thus, the asymptotic Kohn-Sham effective potential at a metal surface is the image potential, and arises soley due to correlations which result from the Pauli exclusion principle. © 1993 John Wiley & Sons, Inc.
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  • 46
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In many situations, the most long-ranged parts of the exchange and correlation holes surrounding an electron cancel one another. Apparently for this reason, local spin density and generalized gradient approximations are more accurate for exchange and correlation together than for either alone. A study is made of the ability of these density functionals, and also the unmodified second-order gradient expansion, to describe various short-range effects in atoms: the correlation contribution to the interacting kinetic energy, the antiparallel-spin correlation energy, and the correction to the random phase approximation. Generalized gradient approximations, constructed with no adjustable parameter from the electron gas of slowly varying density, are found to give results of useful accuracy for real atoms. Prospects are discussed for use of the new functionals to improve the accuracy of electronic-structure calculations. © 1993 John Wiley & Sons, Inc.
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  • 47
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The structural and electronic properties of polysilane, (SiH2)x and polysilene, (SiH)x will be compared with their carbon-based analogues polyethylene, (CH2)x, and polyacetylene, (CH)x, as investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree-Fock (HF) level and by including electron correlation effects up to the third order of Møller-Plesset perturbation theory. The single-particle energy bands have also been corrected for correlation effects applying the electron polaron quasiparticle (QP) method. Electron correlation plays an essential role in stabilizing the gauche conformation of polysilane against the trans form. While the gauche-trans energy difference is about 0.25 mH at the HF level, it will be reduced to about -0.8 mH due to correlation. Also the energy bands and band gaps will be substantially modified in going from the HF to the QP description, resulting in ΔEgap = 9.5 and 11.8 eV for trans and gauche polysilane at HF, and 5.12 and 5.56 eV at the QP level, respectively. Polysilene turns out to be a semiconductor (similar to polyacetylene) for which bond-alternation energetically stabilizes the semiconducting phase (as compared to the equidistant, metallic one) by 1.4 mH at the HF and by 0.6 mH at the MP2 levels, with bond alternations of ΔR = 0.1327 and 0.0865 Å, respectively. © 1993 John Wiley & Sons, Inc.
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  • 48
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, methods of correlation corrections for 1D and 2D periodic systems as well as for 1D nonperiodic polymers are reviewed. These procedures include not only the correlation correction of the total energy per unit cell, but also the recalculation (using the generalized electronic polaron model) of the band structures themselves in the periodic case, as well as the density of states (DOS) in nonperiodic chains. In both cases the inverse Dyson equation was used in an iterative way in the MP/2 and MP/3 level. The programming of the same procedure for the coupled cluster method is in progress. In a number of examples (ground state properties of (SN)X, the gap of alternating trans-polyacetylene, the exciton spectrum of diacetylene, polyglycine and polyalanine, of a cytosine stack, the bulk modules of polyethylene, the hopping conductivity of insulin), we have obtained quite good agreement with experiment, if a good basis set ab initio calculation was supplemented of the correlation correction at the bands or DOS curves, respectively. In the second part of the paper a new method is developed to take into account the simulataneous effect of a static and a periodic time-dependent electric field on the one-electron wave functions and on the total electronic energy of a periodic polymer. The method uses a variational technique (coupled Hartree-Fock equations) instead of perturbation theory for the time-dependent one-electron wave functions. The proper treatment of the potential caused by the field, which in this way does not destroy the periodicity of the polymer, as well as the inclusion of correlation at the MP/2 level of the t-and ω-dependent quasienergy per unit cell is part of the method. Finally, it is shown how one obtains in the usual way the different polarizabilities, second- and third-order hyperpolarizability tensor elements with the help of the derivatives of the quasienergies according to the different field (both static and time-dependent) components. © 1993 John Wiley & Sons, Inc.
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  • 49
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
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  • 50
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is demonstrated that the topological arrangement of atoms can guarantee the existence of stable electronic eigenvalues. These levels are stable in the presence of off-diagonal disorder. Graph theory is used to describe the topological structure of the Hamiltonian. The actual applications are presented in tight-binding approximation. It was found that graph theory can be used even if the atoms have more than one atomic orbital. Also, the localization properties of the topologically determined eigenvalues are studied. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the regime of the molecular-orbital theory, a new approach beyond of the Hartree-Fock approximations is proposed. By an appropriate introduction of the quantum fluctuation into the molecular electronic ground state, the Hartree-Fock equations plus Gaussian fluctuation are obtained. Basically, the procedure proposed consists of an adequate utilization of the Gaussian approximation of the Ising model on a lattice (K.G. Wilson et al., Phys. Rep. C 12, 76 (1974)). It leads to an effective nonlinear Hamiltonian for the molecule for studying nonlinear problems in the molecular electronic structure. © 1993 John Wiley & Sons, Inc.
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  • 52
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An explicitly connected commutator expansion for the average value of an observable in the coupled-cluster theory is derived. Specifically, it is shown that the expectation value of an operator for the state Ψ, related to the Fermi vacuum Φ by the exponential Ansatz ψ = eT Φ, is expressed as a finite commutator series containing the cluster operator T and an auxiliary operator S, defined by a linear equation involving again a finite commutator series in T. The above result is applied to derive the explicitly connected commutator form of the order-by-order many-body perturbation theory (MBPT) expansion for the expectation values and density matrices. We also show how the commutator expansion derived by us can be used as a basis for size-extensive infinite-order summation techniques. An operator technique of eliminating the nonlocal, “off-energy shell” denominators from MBPT expressions is proposed and applied to obtain compact commutator formulas for the expectation values of one- and two-electron operators through the fourth and third order, respectively, and for the correlation energy through the fifth order of perturbation theory. © 1993 John Wiley & Sons, Inc.
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  • 53
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We show that a consistent N-electron theory of a solid can within the Born-Oppenheimer (BO) approximation lead only to normal conductivity or to an insulating state. If one leaves the BO approximation by turning on electron-phonon interaction, one can establish a lower-lying new ground state that has the characteristic properties of the Bardeen-Cooper-Schrieffer (BSC) ground state. We demonstrate that an (N + Nn)-particle Schrödinger equation that describes the motion of N electrons and Nn nuclei of a solid can lead only to phonon-induced superconductivity. © 1993 John Wiley & Sons, Inc.
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  • 54
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: When second-order Jahn-Teller couplings become strong along “streambeds” on potential energy surfaces, instability reflected in negative curvature along a symmetry-lowering distortion coordinate can take place. The point where such negative curvature sets in is usually not a transition state because the gradient of the potential is usually large there. In this paper, it is demonstrated how to use the local energy, local gradient, local Hessian, and knowledge of how quickly the curvature for the symmetry-breaking mode evolves along the streambed (i.e., the derivative of this curvature) to predict how far to move in the symmetry-breaking mode in search of the desired transition state. It is shown that the Hessian matrix evaluated at the symmetry-broken geometry suggested by this analysis has only one negative eigenvalue. Because this analysis is based on a local approximation to the potential, its predictions are, of course, approximate. As such, they only “suggest” the proper direction and magnitude that one should “step” to move toward a transition state. © 1993 John Wiley & Sons, Inc.
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  • 55
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energy levels in sodium atoms with quarks attached to the nucleus are obtained using the configuration interaction method. It is found that the calculated 3p (2P) → 3s (2S) electronic transition in the Na-1/3 quark-atom system is in agreement with that predicted by extrapolation of experimental data. Also, ionization potentials of fractionally charged sodium atoms are discussed. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A survey is given of the resemblance theorems associated with various types of orthonormalization procedures having different kinds of symmetry properties appearing in the current literature. It is shown that the proofs for the minimum properties are closely connected with the fact that the Hilbert-Schmidt norm of an operator is always nonnegative and has the lowest value equal to zero. © 1993 John Wiley & Sons, Inc.
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  • 57
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electric-field maps of certain anions and cations of adenine and guanine have been studied. The results are compared with those of neutral adenine and guanine. In all cases, the geometries were fully optimized using the MNDO molecular orbital method and the fields were computed using Mulliken charges. Reactivities and hydrogen-bonding pattern predicted by electric fields, including those of cations, are in satisfactory agreement with experiment. © 1993 John Wiley & Sons, Inc.
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    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, a general method is presented that allows the derivation of the expansion coefficients of the product of two orthogonal functions provided the generating function is known. For the three classical orthogonal polynomials, the Laguerre, the Hermite, and the Legendre polynomials, the coefficients blmn with φmφn = ∑lblmnφl are derived. © 1993 John Wiley & Sons, Inc.
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  • 60
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chain-length dependencies of 2Ag and 1Bu excitation energies as well as their unexpected inversion observed experimentally in trans-polyenes have so far been explained satisfactorily only in terms of configuration interactions within the standard Pariser-Parr-Pople (PPP) parametrization scheme with at least double excitations involving prohibitively large computational labor for long polyenes. A simpler calculation allowing nonzero differential overlap and employing restricted (first-order) single-excitation configuration interaction with chain-length-dependent screened potential is shown to provide an adequate alternative for the studies of those basic spectroscopic features of polyene excited states. The screening factor is parametrized in accordance with the chain-length-dependent behavior of the electrical polarizabilities of polyenes derived within the standard PPP approximation scheme. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 62
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The ab initio second-order effective valence-shell Hamiltonian that is based on the multireference many-body perturbation theory has been applied to determine adiabatic potential energy curves for the valence states of NH2+ and PH2+ dications. For PH2+, three low-lying quasi-bound states (X1∑+, a3 Π, A1Π) are found and their spectroscopic constants are characterized. In NH2+, no quasi-bound states are found. The potential energy curves for NH2+ are compared with those for PH2+ to understand the difference between these two valence isoelectronic cations. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular calculations are presented using “space-restricted basis functions,” i.e., basis functions that are nonzero only in selected regions of space. The nature of the calculations their justification, and their possible utility to quantum chemistry are described. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 65
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxyl-stretching frequencies (vOH) were estimated for a number of 4-substituted and 2,6-disubstituted phenols using the AM1 all-valence electron molecular orbital approximation. There was significant correlation between calculated and experimentally observed frequencies with better correlation observed with the gas phase as compared with dilute solution-phase values. Significant Hammett sigma-rho relationships were recorded for the 4-substituted phenols, consistent with experimental observations. The vOH stretching frequency associated with the equilibrium structure of 2,6-di-t-butylphenol was calculated to be 15 cm-1 higher than that of phenol and this increase was attributed to steric interactions that act to effectively shorten the OH bond length. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Glutathione (GSH) is the most abundant intracellular thiol. This tripeptide, the γ-L-glutamyl-L-cysteinylglycine, is present inside cells as free sulfhydryl (reduced-form GSH) and in the oxidized-form GSSG, which is converted to GSH by the interacellular enzyme glutathione reductase, in the presence of NADPH. GSH presents numerous functional activities in intermediate metabolism, biosynthesis of macromolecules, transports, cancer therapy, and radioprotection. In all its multifunctional roles, GSH is in the globally monoanionic form GSH- and the three charged groups (the positive amino group and the two negative carboxylate groups) are strongly involved with regard to the environment, particularly, in the GSH-enzyme interactions. Therefore, the intrinsic electrostatic properties of GSH are the first step to the understanding of the molecular mechanism of these interactions. In our study, we present electrostatic properties of GSH, versus various allowed conformations, using ab initio computations. Charges distributions and molecular electrostatic potentials are discussed with regard to the involvement of GSH in its various functions. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Earlier work demonstrated that either a simple fourth-order polynomial or a pair of Morse functions could be fit with high accuracy to model proton transfers across H-bonds. The work is extended here to a systematic set of angular distortions in the H2O‥H+‥OH2 and H3N‥H+‥NH3 systems. So long as the deformation does not impose a left-right asymmetry into the system, either of these types of functions can reproduce ab initio transfer potentials well. But the Morse potentials are superior in that neither stretches nor bends of the H-bonds cause large variation in the parameters. For those modes of angular distortion which introduce asymmetry into the transfer potential, the ab initio data can be accommodated by permitting small variations in two of the parameters in the Morse functions. © 1993 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic and geometrical structures of exchange-coupled open-shell systems such as bis(methylene) methane and the chromium-methylene cation were investigated by the molecular orbital and its extended methods: RHF, UHF, UNO-CAS, RMP, UMP, PUMP, APUMP, UCCSD(T), UNO-CI and QCISD(T). The fully optimized geometries of these species by the energy gradients of UMPn solutions are consistent with the experimental values. The total energies of the lowest and highest spin states were calculated in order to elucidate the effective exchange integrals in the Heisenberg model for methylene and bis(methylene) methane. The spin projection is necessary for the UHF-based wavefunctions in the lowest spin state of the species. The binding energies for the chromium cation-methylene double bond were calculated by both spin-unprojected and -projected methods. It was found that the binding energy between Cr+ and 3CH2 by APUHF QCISD(T) is in accord with the experimental value. © 1993 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 69
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Techniques for computing the multicenter integrals required for molecular calculations for orbitals for which the radial factors are given numerically are described. The methods make extensive use of a numerical algorithm for computing spherical Bessel transforms. The feasibility of using these methods is demonstrated by applying them to SCF calculations for the methane molecule. © 1993 John Wiley & Sons, Inc.
    Additional Material: 2 Tab.
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  • 70
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio electronic structure calculations, carried out in conjunction with rigorous analysis of the resulting wavefunctions, demonstrate that the electronic factors responsible for the unusual stability of the 1,3-dimethylimidazol-2-ylidene carbene are directly related to the substantial σ-backdonation from the carbenic carbon to the adjacent nitrogen atoms. The π-donation is found to play only a minor role, ruling out the presence of significant stabilization due to ylidic resonance structures. The unusually large proton affinity of the carbene is explained by the extra π-electron stabilization of the corresponding 1,3-dimethylimidazolium cation. The MP2/6-311++G** level of theory is found to be sufficient for accurate predictions of the geometries of the imidazol-2-ylidene carbenes. © 1993 John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource