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  • Atomic, Molecular and Optical Physics  (1,863)
  • 1990-1994  (1,617)
  • 1965-1969  (246)
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  • 1
    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 pairon population analysis based on the geminal expansion of pair densities is introduced and applied. As demonstrated by numerical data calculated for a series of simple molecules by the semiempirical MNDO method, the resulting populations provide a new simple means of visualizing the molecular structure. In addition to the reproduction of classical structural formula including the multiplicity of individual bonds, the resulting populations confirm the transferability of bond energies and also provide a simple interpretation of the concept of quantum chemical valence. © 1994 John Wiley & Sons, Inc.
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  • 2
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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
    Notes: In the present article, improved algorithms for evaluating the irreducible representations of the symmetric group associated with an arbitrary partition, such as the orthogonal, the natural, and the seminormal representation, are introduced and the relations among them are discussed. With the new algorithms, a representation matrix for the orthogonal or the seminormal representation is expressed as the product of three matrices, where two of them are the triangular intrinsic matrices depending on the irreducible representation of the symmetric group; the other relating to the permutation can be given explicitly. Furthermore, we give a concise description for the irreducible representations of the symmetric group and reach an interesting conclusion that the conjugation transformation matrix between the orthogonal and the natural representation is the intrinsic matrix of the symmetric group. © 1994 John Wiley & Sons, Inc.
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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: The Möbius transformations recently proposed for integrating functions with a sharp peak close to a single boundary of integration (H.H.H. Homeier and E.O. Steinborn, J. Comput. Phys. 87, 61, 1990) are used to extend Ruedenberg's algorithm for two-center exchange integrals to cases with very diffuse STOs. © 1994 John Wiley & Sons, Inc.
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  • 4
    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: Ketene cycloadditions give predominantly the more hindered product. This has been adduced as evidence for concerted antarafacial cycloaddition in accord with the Woodward-Hoffmann rules. However, stepwise mechanisms, heretofore discounted, are now shown to account for the products in a simple least-motion manner that requires only that rotation in the intermediates be not greatly faster than cyclization. This requirement is amply supported by the literature. © 1994 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 recently introduced second-order similarity index gAB was generalized by using the geminal and/or spingeminal expansion of pair densities that they are derived from. The comparison with previous related studies confirms that the generalization does increase the information content of new indices, especially in providing the information about the difference in the spin recoupling of pure singlet and triplet states of electron pairs. Analogously, as in previous studies, the approach was applied to the analysis of several selected pericyclic reactions and possible mechanistic implications arising from the increased information content are briefly discussed. © 1994 John Wiley & Sons, Inc.
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  • 6
    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 radial Schrödinger equation for the Kratzer molecular potential is equivalent to that of a radial Coulomb problem with an effective (nonintegral) value of rotational angular momentum. The radial Coulomb and the Morse oscillator problems provide different realizations of the algebra so(2,1), whereby the Casimir operators of the Coulomb and Morse oscillator problems are related to the angular momentum quantum number and to the energy, respectively. These relationships permit mappings between the Kratzer molecular potential and the Morse oscillator potential such that the vibrational energy levels of a Kratzer potential with a fixed rotational angular momentum quantum number may be mapped onto degenerate vibrational levels of a set of displaced Morse oscillators. The ground vibrational level of the Kratzer potential is mapped onto the ground vibrational level of a specific Morse oscillator and the remaining (infinite) set of higher vibrational levels are mapped onto degenerate states of displaced Morse oscillators, corresponding to systematic unit increase in the number of bound vibrational levels and successive decrease in equilibrium separation. This behavior is contrasted with that of the finite set of displaced Morse potentials arising as supersymmetric partner potentials to a given parent Morse potential, where there is a systematic unit decrease in the number of bound vibrational levels and a successive increase in equilibrium separation. © 1994 John Wiley & Sons, Inc.
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  • 7
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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
    Notes: It is advocated to carry out an optimization procedure, which is based upon the variational method, in such a way that the optimum values of the variational parameters are expressed as functions of physical constants, such as the atomic number, Z. The three stages involved in this treatment are illustrated by the optimization of nine correlated wave functions, which describe the ground states of atomic two-electron systems. An analysis of the Z-expansions of the total energies associated with these functions leads to the concept of a class of variational functions. The performances of functions belonging to the same class differ only marginally, especially at larger values of Z. Consequently, the concept of class may be used to bring some order in the plethora of variational functions. © 1994 John Wiley & Sons, Inc.
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  • 8
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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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  • 9
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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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  • 10
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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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  • 11
    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 formula of the coefficient aN-K of the eigenpolynomials connected with the zero-order term's coefficient of the eigenpolynomials corresponding the k-order-induced subgraph's molecular fragments has been induced. From this formula, aN-K can be calculated and the contributions of the induced subgraph's molecular fragments to the stability and reactivity of the molecules are revealed. © 1994 John Wiley & Sons, Inc.
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  • 12
    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 screened-Coulomb exchange model used first in the Xα method and later on in Kohn-Sham density functional theory is reexamined. Based on the well-elaborated framework of the local spin-density approximation, we show that this model is not well suited when systems with a finite number of electrons are concerned, because it does not respect the pair-density sum rule. A proper modification of the model is proposed by reformulating it in terms of a screened-exchange hole and ensuring the sum rule for this hole. As a result, it is shown how the static screened exchange in finite systems is accompanied by a conjugate antiscreened one. The possible consequences of this effect on the application of the screened-Coulomb exchange model are discussed. © 1994 John Wiley & Sons, Inc.
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  • 13
    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 concept of internal connectivity associates to each eigenvector of the molecular graph a graph with signed edges. We show that the respective edge signs can be deduced without knowing the graph eigenvectors. The dependence of the edge signs on molecular structure is elucidated. The sign of an edge whose end vertices are νr and νs is found to be the result of separate interactions through all paths connecting the vertices νr and νs. The analysis of the edge signs is particularly simple in the case of acyclic systems. © 1994 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: The energy levels of small titanium oxide clusters [(TiO2)2, and (TiO2H)3, and (TiO2H)2] have been calculated using ab initio SCF methods. Both crystal and relaxed geometries have been considered. Systematic changes in the valencelevel structure resulting from geometry relaxation are found, which may be related to band-gap changes experimentally observed for small titanium oxide particles. In addition, a Ti—OH local surface state is found to be well described within a limited cluster model. © 1994 John Wiley & Sons, Inc.
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  • 15
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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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  • 16
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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
    Notes: The reaction path is shown to be always a gradient line on a potential energy surface (PES) of a molecule. The properties of gradient lines on the PES are elucidated. Correct symmetry conservation rules along the gradient line are derived. The behavior of the gradient line on a PES with different topologies are considered. © 1994 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
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  • 18
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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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  • 19
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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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  • 20
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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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  • 21
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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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  • 22
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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
    Notes: The electron-pair (intracule and extracule) densities of the first-row hydrides may be understood on the basis of the different chemical-bonding schemes in this series. Thus, the LiH pair densities show a strongly ionic nature and may be fairly well described by Li+H-. The CH-pair densities, on the other hand, may be approximated by the promolecular superposition of a carbon and a hydrogen atom with an accumulation of pair density in the internuclear region signifying covalency. In the case of FH, its pair densities show a predominately ionic structure and are closer to those of F- than to those of the promolecular superposition of a fluorine and a hydrogen atom. The slight deformation of longitudinal pair densities observed in FH is largely due to the presence of the H+. © 1994 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: The definition of local hardness is clarified within the Kohn-Sham formulation of the density functional theory. Exact equations for the local hardness of open- and closed-shell electronic systems are derived. It is found that the Kohn-Sham independent particle kinetic energy does not contribute to local hardness. The apparent contradiction with previous work of Parr et al. is cleared up when the local hardness definition is revised. © 1994 John Wiley & Sons, Inc.
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  • 24
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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
    Notes: As the representative point on an electronic energy surface moves along any closed level curve, the corresponding electronic eigenvectors behave as does the normal to a Moebius strip when its foot is moved around the strip. This introduces a topology into the space of eigenvectors that is not envisaged in the definition of Hilbert space, but is fundamental to molecular quantum mechanics. Salient properties of these “non-Hilbertian” vector spaces are established. © 1994 John Wiley & Sons, Inc.
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  • 25
    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 search for a density functional theory (DFT) free of one-electron orbitals needs the development of good kinetic energy functionals beyond the venerable Thomas-Fermi and gradient expansion functionals. With this goal in mind, we previously solved for several atoms and positive ions the Euler equation associated with the exchange and kinetic energy functionals of the nonlocal weighted density approximation (WDA). In this work, we improve the theory by enforcing the correct asymptotic behavior of the exchange potential following a prescription of Przybilsky and Borstel (PB). We have calculated electronic densities, density moments 〈rn〉 (n = -1,1,2) and the density at the nucleus ρ(0), as well as total and exchange energies and chemical potentials for neutral and charged atoms. All those quantities show a better agreement with Hartree-Fock results than those obtained using gradient-corrected Thomas-Fermi functionals. The radial densities, 4πr2(r), show two local maxima at positions close to those of the first two maxima of Hartree-Fock radial densities. This effect, which is absent in the self-consistent densities from the usual gradient-corrected Thomas-Fermi functions, can be interpreted, in our view as an incipient emergence of the shell effect. © 1994 John Wiley & Sons, Inc.
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  • 26
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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: The previously introduced σ and π energy separation is extended to subsystems. σ and π electron donor and acceptor properties of the substituents are tested with a Löwdin population analysis. The influence of the substituents on the σ and π electron energies is calculated for substituted benzene, cyclobutadiene, butadiene, and borazine. A linear relationship is found which shows that an increase of σ electron energy is accompanied by a decrease in π electron energy and vice versa. This allows a classification of substituent groups in terms of reactivity. © 1994 John Wiley & Sons, Inc.
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  • 27
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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: Parr and Chattaraj proposed a principle of maximum hardness for stable molecular structures. Pearson and Palke used ab initio SCF MO calculations for ammonia and ethane to demonstrate the operation of the principle. In this paper, we present ab initio SCF MO results for five isomeric forms of the homoatomic P6 cluster as further support for the principle of maximum hardness. © 1994 John Wiley & Sons, Inc.
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  • 28
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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: We introduce generalized bond orders defined in terms of weighted Kekule valence structures. The weights were determined by the contributions of linearly independent and minimal conjugated circuits in individual Kekule valence structure. When special values for the contributions of conjugated circuits of different size are assumed, one obtains quantities that show considerable similarity to the Pauling and the Clar's bond orders. Pauling bond orders are obtained when one assumes that all conjugated circuits make equal contribution to bond orders. © 1994 John Wiley & Sons, Inc.
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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: Chemical binding is modeled through interatomic charge transfer and accumulation of electron density at the bond center using the concepts of electronegativity and hardness parameters defined for the bond region. The generalized electronegativity and hardness parameters for the up- and down-spin electrons are also defined within the framework of spin-polarized density functional theory, leading to the formulation of covalent binding in molecules in terms of a two-way flow of unpaired electrons between the atoms. The associated energy changes corresponding to these descriptions are shown to provide quite accurate predictions of bond energies for simple heteronuclear diatomic molecules. © 1994 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: An isodesmic energy analysis has been carried out at the MP2/6-31G*//HF/3-21G level for the nonplanar ground state (1) of 1,3,5,7-cyclooctateraene and for two planar forms, one having complete π delocalization (2) and the other having alternating single and double bonds (3). 1 is found to have a considerable degree of stabilization, which is attributed to limited π delocalization. The polyene 3 is the more stable of the two planar forms; it is a transition state in the inversion between two possible nonplanar structures. 2 is found to be a triplet at the Hartree-Fock level and is a critical point on an alternate pathway between the two possible arrangements of alternating single and double bonds in 3. Both 2 and 3 have negative isodesmic energies, indicating the presence of stabilizing factors. Our results for 3 show that an “antiaromatic” system need not necessarily show a net destabilization. © 1994 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: Ab initio configuration interaction (CI) calculations have been performed for the O2 + H2 complex in a trapezoidlike collision arrangement with C2v symmetry. The potential energy surfaces of the four lowest states of this van der Waals complex (arising from the X3 Σg-, a1 Δg, and b1 Σg+ states of the oxygen moiety), as well as the collision-induced b1 Σg+ - a1 Δg electric dipole transition moment (Mb-a), have been analyzed for different CI expansions, using as a reference determinant the restricted open-shell Hartree-Fock (ROHF) function for the ground state of the complex H2(X1 Σg+) + O2(X3 Σg-). The geometry optimized at the ROHF/6-311G** level was refined by a partial optimization at the CI level scanning the intermolecular distance. The equilibrium distances for the X, a, and b states have been found to be a slightly different in the region 3.02-2.98 Å. The larger binding energy of the b1 Σg+ state (2.96 kJ/mol) in comparison with the a1 Δg (2.1 kJ/mol) and ground X3 Σg- states (1.35 kJ/mol) presumably could be explained as resulting from charge-transfer interactions. A good convergence of the calculated transition moment Mb-a for the larger CI expansions (approximately 50,000 configuration-state functions) has been obtained. The calculated collision-induced intensity of the b1 Σg+-a1 Δg and a1 Δg-X3 Σg- transitions in molecular oxygen are in reasonable agreement with recent experimental data for several foreign gases. © 1994 John Wiley & Sons, Inc.
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  • 32
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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
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  • 33
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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: The coefficients of fractional parentage (CFP), treated by tradition as antisymmetrizing multipliers, do not exist as such in the second-quantization representation, because, therein, by definition, there is nothing to antisymmetrize. This by no way means that the above-mentioned representation is less relevant in theoretical atomic spectroscopy than is any other one. On the contrary, all the well-known mathematical apparatus is still successfully applied, alongside with numerous new useful features. Moreover, the analysis presented here yields some results not so obvious from the point of view of the coordinate representation, namely, the consequent application of the second-quantization representation led us to an expansion of many-shell wave functions employing analogs of many-shell CFP, which appeared to be considerably simpler than the traditional CFP expansion. Also, a generalization of Redmond's formula for the case of many-shell CFP and a straightforward method of deriving summation rules for products of CFP are clear in the second-quantization representation and are sketched here. © 1994 John Wiley & Sons, Inc.
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  • 34
    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 method of electron nuclear dynamics (END) developed by Öhrn and Deumens has been applied to the coupling of electronic structure and vibrational motion in the hydrogen molecule with the aim of elucidating and illustrating the characteristic features of this method. © 1994 John Wiley & Sons, Inc.
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  • 35
    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 systematic comparison of the correlation contribution at the level of the second-order polarization propagator approximation (SOPPA) and MP2 to the static dipole polarizability of (1) Be, BeH-, BH, CH+, MgH-, AIH, SiH+, and GeH+; (2) BH3, CH4, NH3, H2O, HF, BF, and F2; and (3) N2, CO, CN-, HCN, C2H2, and HCHO. Fairly extended basis sets were used in the calculations. We find that the agreement with experimental values is improved in SOPPA and MP.2 over the results at the SCF level. The signs and magnitudes of the correlation contribution in SOPPA are similar to those obtained in analytical derivative MP2 calculations. However, it is not possible to say, in general, which method gives the largest correlation contribution or the best agreement with experiment, nor is it possible to make a priori prediction of the sign of the correlation contribution. For the first group of molecules, which have a quasi-degenerate ground state, additional CCDPPA and CCSDPPA calculations were performed and compared with polarizabilities obtained as analytical/numerical derivatives of the CCD and CCSD energies. The CCSDPPA results were found to be in better agreement with other calculations than were the SOPPA results, demonstrating the necessity of using methods based on infinite-order perturbation theory for these systems. © 1994 John Wiley & Sons, Inc.
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  • 36
    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: Single-reference coupled-cluster methods corresponding to various approximate forms of the cluster operator including up to four-electron operators are applied to the simple H4 model system with the aim of numerically studying the attainability and properties of multiple solutions of the systems of nonlinear equations for the cluster amplitudes. Various solutions have been obtained for the excited states 21 A1 and 51 A1. It is documented that there exist nonphysical nonstandard solutions that yield energies very close to the physical ones. A simple way of selecting the physically meaningful solutions is proposed. It is demonstrated that the attainability of nonstandard solutions describing a state of H4 within the framework of a given SE-CC method strongly depends on the cluster structure of that state. © 1994 John Wiley & Sons, Inc.
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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: Recursive formulas are derived for generating the conjugated-circuit polynomials for circuits up to size n = 6, 10, or 14 on regular chain polyhex graphs. These counting polynomials are of use for a simple valence-bond resonance-theoretic derived model: the conjugated-circuit model. © 1994 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: The relative stability of the four tautomeric forms of α-hydroxytetronic acid was calculated with full geometry optimization at the STO-3G, 3-21G, 6-31G*, and 6-31G** SCF levels. Correlation effects were estimated using the MP2 method. Intramolecular hydrogen bonds are found to be of primary importance for the relative stability of the various tautomers, and the same tautomer that in the case of L-ascorbic acid is experimentally observed in the crystal as well as in solution is found to be the most stable one, if polarization functions on the hydrogen atoms are taken into account. It is concluded that even in the gas phase this tautomer predominates in α-hydroxytetronic acid as well as in L-ascorbic acid. © 1994 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: We apply the HH-GLF method, a new simple hyperspherical harmonic method proposed recently by one of us, to directly solve the three-body Schrödinger equation for e+e-e+. Uniformally convergent energy eigenvalues are obtained with only several GLF and the obtained ground-state energy with 200 HH and 6 GLF is -0.26124 au, which is very near the exact value of -0.26200 au. Energy results for maximum global momentum Km ≤ 20 are compared with those from some other hyperspherical techniques carefully, and we find that, in the example of e+e-e+, the HH-GLF method can yield results as accurate as the best available other HH method, but is conceptually simpler and more convenient for practical calculations with a large number of hyperspherical harmonics. © 1994 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: The quantum defect orbital (QDO) method and its relativistic (RQDO) counterpart have been employed in this work to compute oscillator strengths for several fine-structure transitions in members of the copper isoelectronic sequence up to Z = 92. The RQDO results are found to be in quite good agreement with the best estimates derived from other sources, whenever the comparison is possible. © 1994 John Wiley & Sons, Inc.
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  • 42
    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 problems connected with the simulation by the diffusion quantum Monte Carlo method of atomic and molecular systems in the two-dimensional (2D) space have been investigated on the 1 2S and 2 2S states of the hydrogen atom and on the 1 1S and 1 3S states of the helium atom, assuming r-1 Coulomb interactions between the particles. The potential surface of the 1Σg+ state of the 2D hydrogen molecule has been calculated in the range 0.1-1.5 bohr: A dissociation energy of 1.2703 Hartrees is found at the equilibrium distance of 0.3639 bohr. © 1994 John Wiley & Sons, Inc.
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  • 43
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 44
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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: A study of two simple quantum mechanical system, i.e., harmonic oscillator (HO) and hydrogen atom (H) trapped inside the wells is performed. The influence of the spatial confinement on the energy spectra of both systems is investigated. An analytically solvable model of the well potential is presented in the case of the HO. A central and noncentral localization of HO in the well is discussed. The results are used to explain some reported anomalous experimental data on the porphine IR spectrum. The case of hydrogen atom is studied by solving the problem of the electron in the potential V = Z/R cot r/R. An exact formula for the energy is derived for s states. For others, analytically nonsolvable symmetries, a formula with correct asymptotic behavior is proposed. The results are compared with some numerical calculations for spherical rectangular well and an extension to many-electron atoms is also presented. The effect of breaking degeneracy and of ordering energies of H in a different way than in the case of many-electron systems is broadly discussed. © 1994 John Wiley & Sons, Inc.
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  • 45
    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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  • 46
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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: In this article, direct calculations of nonadiabatic NO expansions for the ground-state wave function of H2+ are given, as predicted by Bishop and Cheung. Two methods are presented, i.e., an analogon to the Ahlrichs/Driessler procedure and a gradient method, both originally developed to calculate natural orbitals for two-electron systems. © 1994 John Wiley & Sons, Inc.
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  • 47
    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: Full configuration interaction (FCI) calculations are reported for the closed-shell cyclic polyenes CNHN (N = 6, 10, 14, 18) in the Pariser-Parr-Pople (PPP) approximation, as a function of the hopping parameter β. A wide range of values of β is considered, from a highly correlated situation, β = 0, to a very weakly correlated limit, β = -10 eV. An estimate of the role of higher than two-body connected cluster components was done, through a partial (i.e., limited to two-body terms in the exponent) cluster analysis performed on the FCI wave function. The comparison with the approximate coupled pair theory that accounts for quadruply excited clusters [see P. Piecuch and J. Paldus, Theor. Chim. Acta 78, 65 (1990)] shows a good agreement in the whole range of the hopping parameter, particularly when the contribution of connected triples excitations is also taken into account. © 1994 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: An extended basis set of triple zeta plus polarization quality is employed to carry out configuration interaction (CI) calculations of the three lowest singlet and triplet excited states of benzene. The CI calculation is carried out by taking into account single and double excitations of π and σ electrons. In the CI, composite natural orbitals (CNOs), which are constructed from the natural orbitals of the ground state of ethylene, are used as virtual orbitals. The aim of using CNOs is to reduce the number of virtual orbitals to be used in constructing configuration-state functions, thus cutting down CI dimensions without losing reasonable accuracy. The excitation energies resulting from the CI are in fairly good agreement with experiment. The root mean square of the deviation is 0.22 eV for the six calculated energies and the largest disagreement is 0.37 eV for the third singlet excited state. To obtain better excitation energies by an ab initio calculation, it seems likely that we need to take into account more electron correlation than in the present calculation. © 1994 John Wiley & Sons, Inc.
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  • 49
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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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  • 50
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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: A new class of pilocarpine double prodrugs of the bispilocarpic acid diester type has been investigated. Molecular orbital and molecular mechanics calculations have been done to obtain information about the conformational preferences and general physicochemical properties. Geometries of the structures were obtained with the force field implemented in the CHEM-X program and using the semiempirical quantum chemical program AMPAC and the AM1 method. A correlation between the van der Waals volume, van der Waals area, and log P of the bispilocarpic acid diester structures is observed. The structures of these biscompounds are compared with the structures of the corresponding monopilocarpic acid diesters. Electrostatic and electronic properties of the compounds will be discussed in the context of the interaction between the prodrugs and the hydrolyzing esterase enzymes. © 1994 John Wiley & Sons, Inc.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 52
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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: A numerical method is developed to obtain a sequence of functions converging to the eigenfunctions of the Schrödinger operator H = - ½ Δ + V(r) for V(r) = - Z/r + χ(r), where χ(r) is a continuous and bounded-from-below function for (r ∊ 0, ∞). The criterion of convergence in the convergence in the norm of the Hilbert space L2(0, ∞), which assures the accurate computation of the expected values for a symmetric operator, as we show. The method consists of solving the dirichlet problem inside a box of radius n by the Ritz method, whose convergence in the norm is proved using the compactness criterion. Using a physical argument, we show that the bounded states of the Dirichlet problem converge to those the unbounded system in the norm of L2(0, ∞) as n grows. The method is applied to the potentials V(r) = - Z/r + ari (i ≥ 0) and V(r) = - Z/r + a/(1 + rλ); in each case, we show the numerical convergence of eigenfunctions, energies, and density moments. © 1994 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: Through a new local density approximation to the kinetic energy density functional introduced by us recently, a simple Thomas-Fermi-like scheme for the direct calculation of electron density in atoms is proposed. The calculated density is nonsingular at the nucleus and the energy values are in very good agreement with the corresponding Hartree-Fock results for atoms. © 1994 John Wiley & Sons, Inc.
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  • 54
    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 formulation for calculating frequency-dependent hyperpolarizabilities in the coupled-cluster (CC) theory with the Brueckner orbitals is presented. Our quasi-energy derivative (QED) method is applied, and explicit expressions for response properties up to third order [μ, α(-ω1;ω1), β(-ωσ;ω1, ω2)] are derived. The position of the resonances is determined from the RPA-like equation. It is found that the pole structures are not compatible with ones of the exact response functions. An extension of the coupled Brillouin-Brueckner CEPA-0 by Kutzelnigg to the time-dependent case is also discussed. © 1994 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: Modes of thermal and photochemical cycloaddition of even linear polyenes are examined using the simple concept of activation hardness. The results are in agreement with the well-known Woodward-Hoffman rules. [R.B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry (Academic Press, New York, 1989)]. © 1994 John Wiley & Sons, Inc.
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  • 56
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 57
    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 influence of accepting the hydrogen bond by N(3) of adenine on the conformational preferences of the N(6) substituent in modified nucleic acid base N6-(N-glycylcarbonyl)adenine (gc6Ade) has been modeled by the protonation of N(3). The preferred orientation of the glycylcarbonyl substituent changes on the protonation of N(3). The preferred conformation for the N(3) protonated base is the planar, intramolecular bifurcated hydrogen-bonded structure involving the interaction of the ureido N(11)H with the N(1) of the purine and the O(13b) of the amino acid carboxyl. Another conformation of nearly equal stability, having the internal hydrogen bonding of O(13b) with the N(6)H, is also predicted. Such protonation or hydrogen-bonding-induced conformational flipping may enable the structural reorientation of the anticodon loop required for the functioning of tRNA. © 1994 John Wiley & Sons, Inc.
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  • 58
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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: Recently, it has been shown that the ground-state energy of a quantum many-body system can be written in terms of cumulants. In this article, we show that the energies of excited states can be expressed similarly. These representations are suitable for various approximation schemes, e.g., projection techniques. The explicit use of cumulants ensures size consistency in all approximations. The theory is then applied to the computation of the energy bands of a semiconductor with a diamond structure. As we focus especially on the effects of electron correlations, we consider a simplified model and rederive results obtained previously by the variational local ansatz method. © 1994 John Wiley & Sons, Inc.
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  • 59
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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 important duality problem of phase-space representations of quantum mechanics is discussed and the class of self-dual representations is established. It is shown that self-dual phase-space representations may be endowed with a Hilbert space structure resembling the standard picture of quantum mechanics. A stationary variational principle is then formulated, enabling one to solve eigenproblems of observables directly in phase space. A modified, more usable form of this variational principle is also given together with an example of practical application to the 1D Coulomb system. © 1994 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 helium atom in its ground state, subject to strong magnetic fields, is treated using a variational method. Trial functions consisting of Gaussians with nonlinear parameters variationally optimized are used. The form of these functions is a generalized s-symmetry-state properly modified to admit spatial deformation along the direction on which the magnetic field is applied. © 1994 John Wiley & Sons, Inc.
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  • 61
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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 study of the dimer form of 1,3-dimethyluracil was carried out. The optimum geometric parameters in the monomeric and dimeric forms were obtained using the AM1 semiempirical method. The results were compared with those reported by X-ray diffraction. The energies in the dimer formation and in the intramolecular C—H···O interaction were calculated. Electronic density maps in two and three dimensions were drawn. Several calculated thermodynamic parameters are discussed. © 1994 John Wiley & Sons, Inc.
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  • 62
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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 transition states for unimolecular HF elimination from a series of methylene halides and vinyl halides have been located and properly characterized at the AM1, MNDO, PM3, RHF/6-311G (d, p), and MP2/6-311G (d, p) levels. Whereas the semiempirical MO methods deal well with the structures of the stable molecules, the structural differences between the ab initio and semiempirical transition states are considerably larger. The AM1 and PM3 activation energies appear to be relatively more accurate. © 1994 John Wiley & Sons, Inc.
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  • 63
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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: Within the framework of the AMO-EHF approximation for the Hubbard model, it is shown that the band gap ΔE of an arbitrary regular alternant 1D π-system (polymer), having a singlet ground state, is different from zero, i.e., the ground state is a dielectric one. At least one of the three components of the energy gap - the topological, Δtop; the geometrical, Δgeom; or the correlation, Δcorr (combined via the equation ΔE = {Δcorr2 + (Δtop + Δgeom))2}1/2 - is different from zero. Numerical results for several 1D alternant polymers with conjugated π-systems are given. © 1994 John Wiley & Sons, Inc.
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  • 64
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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: The eigenspectrum of a Möbius graph is a complete inversion of a Hückel graph for odd-numbered monocycles. For even-numbered monocycles, the Coulson-paired levels (xi, - xi) in a Hückel graph become the degenerate levels (x′1, x′1) in a Möbius graph. Here, we present the proof of a general theorem stating that the same eigenspectral relationship is found in a much wider class of graphs. © 1994 John Wiley & Sons, Inc.
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  • 66
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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 modified ionic and covalent valence indices are introduced, defined in the framework of the two-particle density matrix, with respect to the reference state of separated atoms or ions (SAL). They include only quadratic contributions in changes of the molecular charge-and-bond order matrix elements, relative to the SAL. General properties of the modified valence indices are examined and illustrative qualitative results for model systems are presented. Numerical UHF SCF MO valence data for selected diatomic and triatomic molecules are reported and interpreted in terms of the valence saturation effect and the ionic vs. covalent valence competition. A three-orbital valence model of a symmetric transition state of the bond-forming-bond-breaking reaction supports the BEBO model postulate of preservation of the total “bond order.” The model predictions are compared with the UHF numerical values. © 1994 John Wiley & Sons, Inc.
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  • 67
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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 recently proposed a variational method for determining static polarizabilities that uses the ground-state density, rather than the orbitals, of the system for calculations. Thus, the method is different from, and numerically easier than, the perturbation theory approach in which changes in each of the single-particle orbitals have to be obtained self-consistently. For neutral atoms and cations, it leads to results that are comparable to the results obtained by the perturbation theory. In this article, we apply this method, employing Hartree-Fock densities, to obtain polarizabilities for negative ions and show that for negative ions also the results are quite accurate. Thus, the method may prove useful in making quick and accurate estimates of the polarizabilities of more complex systems where orbital based self-consistent calculations become quite complicated. © 1994 John Wiley & Sons, Inc.
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  • 68
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    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first nonrelativistic lower bound to the ground state of the lithium atom is give with E0 〉 -7.47816 au using the method of variance minimization and an extension of Temple's formula. With large Hylleraas-CI basis sets, high-precision upper bounds and isotope shifts are calculated for the three lowest 2S states of the lithium atom, which are best to date. © 1994 John Wiley & Sons, Inc.
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  • 69
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    Topics: Chemistry and Pharmacology
    Notes: A series of basis sets and configuration interaction (CI) wave functions, both of which were constructed so as to systematically approach to the complete set limit and the full CI limit, were used for the ground state of Ne. These calculations yielded an estimated correlation energy of -0.3891 au, which is 99.6% of a recent theoretical estimate of -0.3905 au. The CI value, -0.3821 au, was obtained by SDCI calculation with seven reference configurations by using Slater-type orbitals (STOs) from s to h functions. © 1994 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: High-directional Monte Carlo (HDMC) simulations are performed to investigate the behavior of water molecules in the catalytic sites of human ras p21 proteins and the results are compared among three types of hydrated ras p21 proteins: the normal ras complexed with GDP (normal-p21/GDP), the mutant ras complexed with GDP (mutant-p21/GDP), and the normal ras complexed with GTP (normal-p21/GTP). Also, the original Metropolis Monte Carlo (MMC) simulation is performed on the normal-p21/GDP system. Comparison between HDMC and MMC shows the improved efficeincy of HDMC on this complicated system. Interaction between water molecules is represented by the TIP3P potential function. The solute-water interaction is represented by the Coulomb and van der Waals interactions from the CHARMM force field spliced with TIP3P. The mean-square displacements and dipole correlation functions are reported. We specifically examined the hydrogen-bond interactions existing near the catalytic sites, by the distribution of small water clusters connected by the hydrogen bond and by the average lengths and fluctations of hydrogen bonds between water molecules and solute atoms. Water molecules form different local structures in different types of the p21/GDP(GTP) complexes. The structure and motion of water molecules near the catalytic sites of the proteins are discussed in detail. © 1994 John Wiley & Sons, Inc.
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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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