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  • 1985-1989  (1,638)
  • Computational Chemistry and Molecular Modeling  (1,638)
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 125-139 
    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 close scrutiny of the relationship between geometric constraints and motion in a system of water molecules allows the prediction of a role of solvent dynamics in the solvent-mediated interaction between two solutes and between different parts of one solute. The predicted mechanism is cooperative, and its operation is widely supported by experiments (not only by our group) in which solvent dynamics were modulated by isotopic or cosolvent perturbations. More recent experimental work by our group and simulation work of MCY water by Fornili et al, strengthens this evidence and visualizes how an immobilized water molecule can “seed” or “pin” a high-connectivity patch of H bonds. Involvement of the mechanism in pattern-specific, solvent-mediated driving forces of significant size (additional to those due to electrostatic recognition determinants) and in functional proton-transfer is inferred from available evidence.
    Additional Material: 7 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 141-151 
    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 complete understanding of the role of carbohydrates in biological systems is to a large extent dependent on the information available about the equilibrium mixture and about the preferred conformation of the carbohydrate molecules in solution. The conformational analysis offers a tool which can determine all possible conformations which influence the solution behavior of carbohydrates.This paper attempts to survey the progress in the theoretical conformational analysis of saccharides in solution. The conformational analysis will be discussed in detail both with respect to the strategy for the investigation of conformational properties but also with regard to the quality of the method used for calculations of the energy of the isolated molecule and free energy of solvation.Finally, examples will be given to illustrate how the methods of conformational analysis can be used to estimate the solution behavior of cyclic model compounds of carbohydrates 2-methoxytetrahydropyran, monosaccharide D-glucopyranose, and two disaccharides; β-maltose and β-cellobiose.
    Additional Material: 5 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 153-165 
    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 effects of a proximate condensed environment as the solvent and cellular structured patterns (biopolymers, membranes, etc.) play an important role in determination of the courses of molecular processes in biology. We present here the background of methods developed for such an environmental effects estimation combining the continuum and discrete models. Their applications within theoretical studies into the mechanisms of carcinogenic action of alkylating N-nitrosocompounds are shown. The results given cover four different areas, namely the quantitative structure-activity relationship, mechanistic studies into their metabolic activation reactions, interactions of the ultimate carcinogens with DNA, and finally their genetic consequences.
    Additional Material: 2 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 167-180 
    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: There are significant differences between the conditions for chemical and biochemical reactivity. There-fore, models for treating chemical reactions are mostly not suitable for investigating interactions and transformations of bio(macro)molecules. Common features of numerous processes occurring in vivo and in vitro (such as the role of water, ions, and colloids and the significance of Helmholtz energy surfaces) are outlined. Some characteristics of a model suitable for studying van der Waals interactions between biomacromolecules, based on Brownian dynamics and the Lifshitz theory, are described.
    Additional Material: 3 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 215-221 
    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
    Additional Material: 3 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 241-252 
    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
    Additional Material: 4 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 223-239 
    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: Recent studies performed in our group on a classical problem of quantum chemistry, with strong implications for theoretical biochemistry and pharmacology, are here summarized. Ab initio descriptions of noncovalent interactions, and in particular H bonds and acid-base couples, have been reexamined using as novel tools the decomposition of ΔE with the inclusion of CP corrections and a further decomposition of the ΔE components into group contributions. Some results of systematic analyses performed over H-bonded dimers are reported, supplemented by a successful application of this approach to a problem of noticeable economic importance (the identification of catalysts for the industrial synthesis of tensioactives). A new feature, presented here for the first time, is the extension of the CP-corrected decomposition of ΔE to bimolecular interactions in solution.
    Additional Material: 8 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989) 
    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
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 425-432 
    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: One of possible approaches to the CI method is based on Boys bonded functions which can be generated in a systematic way forming an independent set of high internal symmetry. The main disadvantage of bonded functions is their nonorthogonality. In this paper a scheme is proposed for passing to orthogonalized set of bonded functions together with the appropriate algorithm for the transformation of the energy matrix H. The orthogonalization matrices are shown to reflect high symmetry of the canonical set of bonded functions, and in what follows they can be defined by short vectors. Moreover, the orthogonalization transformation can be handled in a blockwise manner.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 433-439 
    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 show that even when the nuclear motion is quantum mechanically described, the energy of a molecular state can be exactly expressed as a sum of terms, each one related to a nuclei of the molecule. This is a generalization of a similar result obtained by Politzer and Parr for the case in which the nuclei are in fixed positions. In fact, the nuclear motion is exactly taken into account through a modified screening function derivative evaluated on each nucleus of the molecule. We discuss how the screening function derivatives corresponding to the fixed nuclei case relate to those which take into account the nuclear motion. A simple relation between these two arises in the so-called crude Börn-Oppenheimer approximation.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 461-470 
    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 combinatorial derivation of the product of the class of three cycles, [(1)N-3(3)]N with an arbitrary class operator of the symmetric group SN is presented. The form of this result suggests a conjecture concerning the expression of the general class operator product in terms of a relatively small number of reduced class coefficients. The conjecture is applied to the determination of the products of [(1)N-4(4)]N, [(1)N-4(2)2]N, and [(1)N-5(5)]N with arbitrary class operators. General expressions for the reduced class coefficients of the simplest type are obtained.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 471-481 
    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 interaction range analysis of electronic structures has been developed with density matrix approximation techniques. A successive error analysis of electronic interaction of differing ranges reveals clear pattrns. Aromatic hydrocarbons and heterocycles are used as examples. The developed technique is proposed to be used as an illustrative tool for the description of delocalization and as a guide for studies where delocalized systems are modeled by small subsystems.
    Additional Material: 6 Ill.
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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: Structural parameters of a set of five thiocarbonyl molecules in the lowest nπ* states are calculated by using a generalized orbital optimization algorithm (a variant of the orthogonal gradient method) in an INDO MCSCF framework. Transition energies, singlet-triplet splittings, planar inversion barriers, and dipole moments in nπ* states of different spin multiplicities are reported. Predicted structural features agree reasonably well with available experimental or theoretical data. Some interesting trends are noted in the computed inversion barrier heights, singlet-triplet splittings, and dipole moments in nπ* states.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 513-518 
    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 local energy is examined as an indicator of the accuracy of approximate wave functions for the ground state of helium. It is observed that at a given point (1) an inaccurate local energy may or may not correspond to an inaccurate value of the wave function or probability density, but (2) a value of the local energy within 0.1 a.u. of the ground-state energy corresponds to a value of the approximate wave function or probability density within about 10% of that for the ground-state wave function.
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  • 15
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 519-549 
    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 contributions of the triple and quadruple excitations to the fifth-order perturbation energy for the perturbation configuration interaction using localized orbitals (PCILO) method are derived. This completes the development of a fifth-order constant denominator perturbation theory initiated in a previous paper [5] with the single and double excitations. This theory is tested on molecules containing strained ring geometries, stretched bonds, strongly polarized bonds, and delocalized pi systems: cases where the starting zero order reference wave function poorly describes the system. Although the perturbation expansions turn out to be slowly convergent, the Padé approximant taken from an energy series which itself is constructed from Padé approximants provides results accurate to within a few kilocalories/mole of benchmark calculations. Computational times as in the original PCILO procedure remain proportional to N3, where N is the number of bonds.
    Additional Material: 11 Ill.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 551-566 
    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 explicit formulas for the evaluation of the Hamiltonian matrix elements are presented. The calculation of the integral coefficients is independent of both the nature of the orbitals and th spin coupling schemes. It is fully automatic and only dependent on the number of doubly and singly occupied orbitals. Further-more, the symmetric group representation matrices are not needed, and the N! problem can be avoided.
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  • 17
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 567-575 
    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 configuration-averaged Fock (CAHF) operator is developed within the restricted open-shell formulation (ROHF) for the case of any number of open-shell orbitals containing any number of electrons. The resulting orbitals and energies should be useful for those cases where there is a great degeneracy of states, either preventing self-consistent field convergence, or negating the value of a pure state description itself. This procedure is compared with the hyper-Hartree-Fock method of Slater and the grand canonical Hartree-Fock ansatz of Abdulnar, Linderberg, Öhrn, and Thulstrup.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 577-580 
    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
    Additional Material: 1 Ill.
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  • 19
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 581-581 
    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
    International Journal of Quantum Chemistry 35 (1989), S. 585-592 
    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 unitary operator U, which can transform the Fock space of a three-dimensional isotropic harmonic oscillator into the space in which the Hamiltion of three coupled oscillators is diagonized, is found. The coordinate representation of U is presented and is used to directly derive the wave function of the energy eigenstate of the coupled oscillators.
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  • 21
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 593-612 
    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 practical method for generating irreducible matrix reps of point groups and a concise formula about projection operators are proposed. By using this formula as well as versatile classification schemes, the symmetry adaptation of a many-electron system is simplified. A unified algorithm and program of symmetry adaptation of spin-free space have been developed.
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  • 22
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 613-628 
    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 describe how the local equilibrium electronic structure in a large system can be determined by using the recursion method of Haydock. By applying this method to the polyacetylene molecule, the energies of each atomic site and the forces acting on each bond are estimated and the equilibrium structure near the end of polyacetylene is determined. It is found out that the degree of localization of π electrons and the bond length alternation become stronger near the end of the polyacetylene molecule.
    Additional Material: 7 Ill.
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  • 23
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 679-685 
    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 self-consistent multiple scattering Xα (MSXα) method is used to arrive at the electronic structure of a helium-trapped vacancy in aluminum using a cluster model. The calculated orbital energies, electron density, and charge distributions are compared with the available theoretical results. It is shown that helium trapping causes the reshuffling and upward movement of the cluster levels (with vacancy).
    Additional Material: 2 Ill.
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  • 24
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 665-677 
    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: N-Nitrosamines are believed to act as carcinogens by alkylating DNA in their ultimate carcinogenic forms which are produced by metabolic activation. Alkylation at certain oxygen sites in DNA, described as “promutagenic,” appears to be of particular significance for mutagenesis and cancer, as indicated by experimental findings. This theoretical study deals with two factors involved in the alkylation of these promutagenic oxygen sites by N-nitrosamine ultimate carcinogens. The first is the competition between alkylation at the promutagenic O6-guanine and O4-thymine sites and that at the nonmutagenic N7-guanine site, which is here related to the degree of participation of cationic ultimate carcinogens as compared with neutral ultimate carcinogens. Parent dialkylnitrosamines are classified structurally according to their degree of cationic ultimate carcinogen participation and preference for promutagenic alkylation. The second factor is the thermodynamic facileness of alkylation at the O6-guanine and O4-thymine sites. Heats of alkylation by candidate ultimate carcinogens are calculated here for numerous parent dialkylnitrosamines. Finally, these two factors are jointly considered in an attempt to correlate them with experimental carcinogenic potency of the parent nitrosamines. Out of the patterns of correlation observed, light is shed on mechanistic factors likely to be involved in the modulation of parent carcinogenic potency.
    Additional Material: 5 Ill.
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  • 25
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989) 
    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
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 687-700 
    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 consider the Hückel approximation to the π-electron spectrum of molecules which are built by linking a number of identical fragments to a central atom in an identical manner. The Hückel matrix H of the composite molecule (or equivalently the adjacency matrix of the molecular graph) is simply related to the Hückel matrix h of the fragment and a vector \documentclass{article}\pagestyle{empty}\begin{document}$ vec{f} $\end{document} which encodes the bonding of a fragment to the central atom. The eigenvalues and eigenvectors of H are obtained from those of h. The orbitals of the composite molecule are of three types: (1) a molecular orbital of the fragment localized on one of the fragments, (2) a molecular orbital of the fragment spread over more than one fragment, and (3) orbitals spread over the entire molecule including the central atom. The orbital energies Λ of the first two types of orbitals are same as the orbital energies λ of the fragment. Energies of the third type of orbitals separate a subset of orbital energies of the fragment and, barring accidental degeneracy, they are distinct from all orbital energies of the fragment. It is only through the third type of orbitals that the composite molecule manifests itself as a new entity rather than an aggregate of noninteracting fragments. It is shown that the graph group of H fails to explain its degeneracy if any eigenvector of the subgraph, not orthogonal to the connection vector \documentclass{article}\pagestyle{empty}\begin{document}$ vec{f} $\end{document}, belongs to a degenerate manifold of h. This solves a long-standing puzzle regarding degeneracy in the Hückel spectrum of triphenylmethyl.
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  • 27
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 801-811 
    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 aspects of R matrix theory that derive from a finite element method implementation of a hyperspherical coordinate formulation of the quantum mechanical three-body problem are analyzed with a view of designing suitable algorithms. Propagation of the R matrix from a hyperspherical surface to asymptotically appropriate wave fronts is given special attention.
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  • 28
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 887-893 
    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: New solutions for reaction-diffusion equations, called singletons and polytons, are found to be of fundamental significance. They exhibit properties of self-formation. In the presence of nonlinear saturation effects their evolutions in time have pronounced maxima. Information about details of an initial spatial distribution is found to be carried along in time with the dynamic process of forming a growing singleton and to be restored at a later stage.
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  • 29
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 36 (1989), S. 1-3 
    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
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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: Compton spectra excited by 241Am radiation have been measured on thin single-crystal slices with the scattering vector oriented parallel and perpendicular to the pair of hydrogen bonds in the (HCO3)22- dimers as well as to the plane of the dimers. the reciprocal form factors extracted from the spectra are strongly anisotropic with two extra zero passages in the direction of the hydrogen bonds. the results are in very satisfactory agreement with theoretical data that are calculated for the dimer with and without additional point charges simulating the neighboring ions in the crystal. The calculations have been done with Gaussian basis sets of double-zeta quality within the Hartree-Fock approximation. The theoretical reciprocal form factors of the monomer HCO3- and the dimer show only small differences in the directions perpendicular to the hydrogen bonds, whereas the differences in the hydrogen bond direction are remarkable and account for the experimentally observed features. In comparison to this effect the influence of the neighboring ions by their charges is much smaller, which is even true when taking into account the influence of the neighboring dimers in the crystalline stack in more detail by symmetrical orthogonalization of the combined wave functions. The same orthogonalization procedure applied to a pair of monomers in the spatial arrangement of the dimer yields a reciprocal form factor that in the experimentally reliable range above 1.8 Å accounts for most of the dimerization effect in the direction of the hydrogen bonds. Thus also for this prototype of paired hydrogen bonds (cf., carboxylic acids, DNA, and RNA), it confirms our earlier experimental finding on liquid water now unambiguously, namely that, in the same way as the cohesion in ionic crystals, hydrogen bond formation in bulk matter is distinctly dominated by electrostatic attraction, which is compensated by repulsion owing to the Pauli principle.
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  • 31
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 36 (1989), S. 15-18 
    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 problem of the validity of the mass-velocity operator in computational quantum chemistry is discussed. The opinion that the mass-velocity operator is incorrect is shown not to be well founded.
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  • 32
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 36 (1989), S. 5-14 
    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 ab initio calculation has been performed with the addition pathways of HLi and its dimer to acetylene at the RHF/3-21G basis set. It shows that the reaction mechanisms of these two reactions are rather similar. In either of two reaction pathways, there is a meta-stable molecular complex near the isolated reactant state. This kind of addition can be treated approximately as the unimolecular reaction in which the molecular complex rearranges into the product. We have estimated the activation entropies and the statistical A factors of these two reactions by the use of RRKM theory. Frontier molecular orbital analysis of these two transition states reveals their HOMOS to be formed from both HOMO-LUMO and HOMO-HOMO interactions.
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  • 33
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 36 (1989), S. 35-48 
    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 spin-free polynomial representation of antisymmetrized geminal products is presented for several cases. In particular, products of identical geminals, which possess different spin multiplicity, are considered. The cases of singlet geminals, singlet geminals with one or two triplet geminals coupled to the lowest possible spin multiplet, and triplet geminals coupled to an arbitrary multiplet are considered in detail, and explicit polynomial representation is given.
    Additional Material: 1 Ill.
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  • 34
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    International Journal of Quantum Chemistry 36 (1989), S. 19-33 
    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 set of 18 aryltriazenes and the antitumor drug DTIC, whose mutagenicity has been determined in the Ames test, has been studied using octanol-water partition coefficients (P) as a measure of relative hydrophobicity and MNDO molecular orbital energies and electron distributions to account for variation in their electronic characteristics. A good structure-activity relationship was found using log P in combination with either εHOMO, or qHOMO, where the latter is defined as the HOMO electron density on N1 of the triazene. The dependence of mutagenicity on hydrophobicity is similar to what had been found earlier by Venger, Hansch, Hatheway, and Amrein [4] for the aryltriazenes alone using σ+ as a descriptor of electronic characteristics. A consistent role for electronic factors is found using σ+, εHOMO and qHOMO, namely, increased mutagenicity is associated with structural variations that increase the electron donating ability of the triazenes.
    Additional Material: 4 Ill.
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  • 35
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    International Journal of Quantum Chemistry 36 (1989), S. 49-60 
    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 perturbative SCF CI treatment to obtain energy levels of coupled oscillator systems is proposed. The method uses the virtual SCF basis set, and the SCF equations are solved by means of a perturbative treatment that provides the diagonal matrix elements involved in the CI calculation. The off-diagonal matrix elements are calculated using a commutation relationship derived from exact quantum theorems. Numerical results for several systems are obtained and compared with those from others SCF, SCF CI, and variational treatments.
    Additional Material: 8 Tab.
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  • 36
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    International Journal of Quantum Chemistry 36 (1989), S. 61-65 
    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 Riccati equation is shown to be suitable for obtaining implicit approximate analytic expressions for the eigenvalues of quantum-mechanical systems. The Hamiltonian operator H = (1/2)p2 - (Z/r) + λr2 is used as a test example, and the resulting formulae are modified to deal with the Zeeman effect in hydrogen.
    Additional Material: 2 Tab.
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  • 37
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    International Journal of Quantum Chemistry 36 (1989), S. 67-91 
    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 order to apply the TDHF equations in the matrix form we established previously, we conceive a model for which the static Hartree-Fock equations are rigorously solved; so we study problems only linked to the TDHF method, independent of any further approximation (for example, of LCAO type). This model is made of a spherical box containing electrons subject to a particular potential. We solve, numerically, the TDHF equations at different orders, and we calculate the third order hyperpolarizability tensor. We then compare our results with the ones given by a variation-perturbation method.
    Additional Material: 4 Ill.
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  • 38
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    International Journal of Quantum Chemistry 36 (1989) 
    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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  • 39
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    International Journal of Quantum Chemistry 36 (1989), S. 119-125 
    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: Static longitudinal polarizabilities αzz and cubic hyperpolarizabilities γzzzz are obtained by the FPT-INDO method for finite chain C4N+2H2N+4 (N = 1-15) models of polydiacetylene and polybutatriene. For the acetylenic structure the onset of saturation first becomes evident at about C14—C18 for the linear polarizability and C22—C26 for the cubic term. The values per C4H2 unit are accurately extrapolated to the infinite chain limit. In the case of the butatrienic structure, no saturation is observed through C54.
    Additional Material: 3 Ill.
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  • 40
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    International Journal of Quantum Chemistry 36 (1989), S. 105-118 
    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 effect of molecular interaction on the O—H stretching force constant of methanol (MeOH) is reported for its associated species. The various electron donors (D) and acceptors (A) considered include organic molecules such as methanol, dimethylether, acetone, acetonitrile, dimethyl formamide, pyridine, and ions such as F-, Cl-, Li+, and H+. The variation in the O—H stretching force constant of MeO—H…D species on interaction with the electron acceptor such as in the species is explained on the basis of the cooperativity effect. (CE). The effect is discussed in terms of the relationship CE = (ΔF/F) × 100, where ΔF is the reduction is force constant of the hydrogen-bonded O—H stretching mode of the associated methanol species MeOH…D when the lone pair electrons on oxygen of the methanol molecule are involved in hydrogen bonding with A, and F is the hydrogen-bonded O—H stretching force constant of the species when the lone pair electrons are free. The cooperativity effect (CE) is found to increase with increasing electron acceptor and electron donor capacities of A and D. The calculated force constants are compared with the experimental results.
    Additional Material: 5 Ill.
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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 bivariational Hartree-Fock scheme for a general many-body operator T is discussed with particular reference to the complex symmetric case: T† = T*. It shown that, even in the case when the complex symmetric operator T is real and hence also self-adjoint, the complex symmetric Hartree-Fock scheme does not reduce to the conventional real form, unless one introduces the constraint that the N-dimensional space spanned by the Hartree-Fock functions φ should be stable under complex conjugation, so that φ* = φα. If one omits this constraint, one gets a complex symmetric formulation of the Hartree-Fock scheme for a real N-electron Hamiltonian having the properties H = H* = H†, in which the effective Hamiltonian Heff (1) may have complex eigenvalues εk. By using the method of complex scaling, it is indicated that these complex eigenvalues - at least for certain systems - may be related to the existence of so-called physical resonance states, and a simple example is given. Full details will be given elsewhere.
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  • 42
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    International Journal of Quantum Chemistry 35 (1989) 
    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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  • 43
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    International Journal of Quantum Chemistry 35 (1989), S. 1-1 
    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
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  • 44
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    International Journal of Quantum Chemistry 35 (1989), S. 103-112 
    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: Results of a Monte Carlo simulation of the hydration of caffeine (1,3,7-trimethyl-2,6-dioxipurine) and three different configurations of its stacked dimer are presented. Simulations were performed in a cluster approximation; the system contained 200 water molecules. The nature of the stabilization of caffeine stacking associates in water was considered. A hydrophobic behavior of methyl groups during association of caffeine molecules in water is shown. Results of the simulation of caffeine hydration are compared with experimental data from the study of the system of caffeine-water.
    Additional Material: 5 Ill.
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  • 45
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    International Journal of Quantum Chemistry 35 (1989), S. 91-102 
    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 conformational properties of some nucleotide sequences result in their ability to bind specifically some ligands or tobe recognized by specific proteins. In order to investigate the dependence of conformational behavior of the DNA duplex on nucleotide sequence, we analyzed the interaction energy of nucleic acid bases as a function of conformational parameters and base sequence. Extended regions of minimum energy values were found for different sequences. Although these regions (valleys) largely overlap, each one shows specificity for a particular sequence. This suggests that a specific pathway of changes in conformational parameters exists for each sequence. the changes may be accompanied by considerable shifts (2-3 Å) of the atom positions and an only slight variation (1-2 kcal/mol) of energy. Even small shifts in other directions can cause a drastic energy increase. For some nucleotide sequences, the energetically preferable conformations are the B-like ones (e.g., ApA, TpA), whereas for others the A-like ones are preferable (e.g., GpG, ApT). IN general, Pyr-Pur sequences have a tendency to a larger τ and smaller H and D than Pur-Pyr sequences. A large body of experimental data on nucleic acid structure in fibers and in solutions can be explained by results obtained.
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  • 46
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    International Journal of Quantum Chemistry 35 (1989), S. 113-124 
    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 biological system usually operates under nonstable values of relevant parameters, such as pH, temperature, and ionic strength. The question therefore poses whether such fluctations do affect its relevant processes. Experimental studies on the role of random temperature fluctuations on functional encounters of biostructural polymer molecules, and consequent self-assembly of supramolecular structures, have evidenced an additional, noise-induced order of these structures. This type of effect is the result of nonlinearity in physical systems, and the case of a biosystem is especially interesting. As recent experiments have shown, spinodal decomposition resulting from thermodynamic instability may favor the onset of the supramolecular ordering process. If the random fluctuations of temperature are imposed in such conditions of thermodynamic instability, their ordering effect is further enhanced.
    Additional Material: 7 Ill.
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  • 47
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    International Journal of Quantum Chemistry 35 (1989), S. 193-203 
    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: Nucleoside antibiotics, which result as a consequence of minor modifications in pyrimidine and purine nucleosides, exhibit a wide variety of antiviral, antibacterial, antitumor, and cancerostatic properties. The conformational properties of a number of these antibiotics have been investigated by using the quantummechanical PCILO method, and the results indicate that the nucleoside antibiotics and their parent nucleosides have very similar conformational preferences. This similarity is strikingly marked in the situations which prevail in an aqueous medium. As a result, these antibiotics easily get incorporated in growing chains of RNA and DNA by mimicking their parent nucleosides and then bring about the inhibition of protein, RNA, or DNA syntheses. The experimental observations corroborate these deductions, and thus a correlation has been obtained between the conformation and the biological activity of nucleoside antibiotics; it is the striking conformational similarity between the nucleoside antibiotics and their parent nucleosides which gives rise to their biological activity. The PCILO investigations carried out on two 3-deazapurine nucleosides demonstrate that the converse of the above correlation also holds true.
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  • 48
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    International Journal of Quantum Chemistry 35 (1989), S. 205-214 
    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, quantum chemical methods have been used to develop an interaction model for the binding of benzoic and salicylic acids and phenols to cyclooxygenase, leading to their antiinflammatory action. The model is based on a regression analysis of the energy of the highest occupied molecular orbital with the potency of the active substances to inhibit prostaglandin production in mouse macrophages and on an analysis of the frontier orbital charge distributions and electrostatic potentials of active and inactive compounds. The model suggests that binding is controlled by an electrostatic orientational factor and a charge transfer or polarization contribution. The observed relative potencies of the phenols and acids can be rationalized with the help of the proposed interaction model.
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  • 49
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    International Journal of Quantum Chemistry 35 (1989), S. 181-191 
    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: Quantum chemical studies (INDO-RHF-SCF) have been made for the resting state active sites of three closely related heme proteins, cytochrome c peroxidase (CCP), metmyoglobin (MMB), and catalase (CAT). The relative energies of the germane sextet, quartet, and doublet spin-states of each active site were calculated. Both CCP and MMB have similar heme units, consisting of an Fe(III)-protoporphyrin-IX with an imidazole and water as axial ligands. Our calculations show that the larger doming of the porphyrin, greater out-of-planarity of the iron, and the shorter iron-water distance in MMB leads to a sextet ground state with a low-lying quartet state. By contrast, the order of these two states is reversed in CCP, when a neutral imidazole is used as the endogenous axial ligand. An imidazolate ligand, on the other hand, which is an extreme representation of the H-bonding believed to occur in CCP with a nearby aspartate residue, leads to a sextet ground state with a low-lying quartet state. Assuming at least a partially anionic ligand in the intact protein, it follows that the quartet contribution to the ground state properties will be larger in CCP than in MMB. These predictions are consistent with the observed differences in the temperature-dependent magnetic susceptibility for these two proteins. The present results suggest that the experimentally observed Mössbauer resonance spectra of CCP should be reinterpreted in terms of sextet and quartet state contributions to the electric field gradient. Calculations for catalase, which has a single phenolate ligand, result in a sextet ground state with a low-lying quartet state consistent with available Mössbauer and magnetic susceptibility data. Our calculations of the Im- form of CCP show that it more closely resembles CAT. Thus, the effect of proton transfer in CCP can account at least in part for the similarities between CCP and CAT function. Minor differences in ground spin-state and electronic properties calculated for CCP and MMB, however, cannot explain why MMB does not have significant peroxidase activity. The different functions of MMB and CCP must then be due in part to other known differences in their protein environment such as polar residues around the distal ligand binding pocket of CCP, which are absent in MMB, and could help its transformation to an active oxidizing state.
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  • 50
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    International Journal of Quantum Chemistry 35 (1989), S. 253-253 
    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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  • 51
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    International Journal of Quantum Chemistry 35 (1989), S. 255-255 
    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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  • 52
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    International Journal of Quantum Chemistry 35 (1989), S. 257-266 
    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: Wave function tails are analyzed quantitatively by investigating the dependence of exterior electron density (EED) on basis sets; the EED is defined as the integrated electron density outside the repulsive molecular surface. Ab initio MO calculations with large scale basis sets were performed to establish the benchmark order of EED values for valence orbitals of some simple molecules. It is found that very popular basis sets, such as 4-31G, which are determined by energy optimization, are inferior in describing the wave function tails to some similar size basis sets, such as MIDI-4, which are obtained by least-squares fit to near Hartree-Fock atomic functions. Further the EED values for atomic 2s functions are shown to be unfavorably smaller than those for atomic 2p functions when the same value is used for the exponent α in the GTO basis sets. This indicates that the frequently used constraint αs = αp is not appropriate for describing wave function tails with medium-size basis sets. Deficiencies in the energy-optimized basis sets are found to become more serious for molecules including heavier atoms.
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  • 53
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    International Journal of Quantum Chemistry 35 (1989), S. 277-282 
    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 idea of a Franck-Condon (FC) operator is introduced, and its normally ordered form is obtained through the newly developed technique of “integration within an ordered product of operators (IWOP).” It is shown that the FC operator leads to a new approach for the calculation of FC factors. The results of existing theories are viewed, and the connection between the FC operator and the “squeeze-operator” is pointed out.
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  • 54
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    International Journal of Quantum Chemistry 35 (1989), S. 267-275 
    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 simple model for calculating the interatomic interaction energies in the electron gas approximation. We use a generalization of the supermolecular electronic density which includes a density overlap term. We present numerical calculations for the He-He interaction as an illustration of the method.
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  • 55
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    International Journal of Quantum Chemistry 35 (1989), S. 283-295 
    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 explicit expression for the dressed potential of a polyatomic molecule, in the adiabatic approximation, is derived. This expression clearly shows the importance of the nuclear charge-mass ratio (NCMR) for the change of potential due to photon fields. It is found from a simple calculation that the 1H atom is the only atom having an abnormal NCMR value; all other atoms have similar, or the same, values. This means that only those molecules containing a 1H atom should be strongly affected by fields. On the basis of this new physical insight, we postulate two rules, which enable us to classify molecules, with respect to their response to intense photon fields, into three classes: high-sensitive, low-sensitive, and insensitive molecules. Qualitative verification is also given by using water isotopes.
    Additional Material: 4 Ill.
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  • 56
    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 extensive theoretical calculation of the tetraphenyldithiapyranylidene DIPSφ4 molecule by means of the MSXα method is presented. The results obtained permit one to interpret the optical spectrum and the XPS spectrum as well as the charge transfer between DIPSφ4 and planar molecular semiconductor compounds such as TCNQ. To our knowledge, this is the first theoretical determination to date of such a charge transfer ratio from separate parameter-free calculations on large-size molecules.
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  • 57
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    International Journal of Quantum Chemistry 35 (1989), S. 359-359 
    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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  • 58
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    International Journal of Quantum Chemistry 35 (1989) 
    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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  • 59
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    International Journal of Quantum Chemistry 35 (1989), S. 361-372 
    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 “apparatus” operator approach is presented for the extension of the density functional theory of Hohenberg, Kohn, and Mermin. Using the Mermin entropy principle, a one-to-one correspondence is established between the density matrix for the system and the electron charge density for a finite-temperature system in the presence of an apparatus. In the zero-temperature limit in the absence of an apparatus, the Hohenberg-Kohn theory is recovered. The central aspect of this new density functional theory is that the principle of maximum entropy is applied to the system plus its surroundings under the additional constraint that the electron charge density is given. The system is treated as a subsystem of a composite system and is not necessarily in the equilibrium state as in the Mermin theory. As an example, it is shown how, in principle, excited states are encompassed by the theory.
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  • 60
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    International Journal of Quantum Chemistry 35 (1989), S. 373-383 
    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 Herndon-Simpson model for a particular catacondensed polyphene chain is considered as a nontrivial many-body Hamiltonian, defined on a space with a basis of orthonormal Kekulé structures. An Explicitly correlated cluster expanded resonance-theoretic wave function is described for this model, and its quality is judged by calculation of the standard deviation for the energy expectation. The quality is found to be high. Indeed, for a particular parameter ratio within the range of experimental interest, the wave function ansatz is found to be exact. This very accurate solution is then used to gauge the quality of the common ansatz with equally weighted Kekulé structures, and it is found to be reasonably good.
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  • 61
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    International Journal of Quantum Chemistry 35 (1989), S. 385-393 
    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: Double-well potential energy surfaces for hydrogen bonding in crystalline vitamin B6 have been associated with molecular environmental effects. New calculations, involving improved representations of a fragmentation model, include the introduction of a second pyridoxinium chloride system within the model in a “dimer-like” configuration. The new results confirm the double-well potential and the prediction of the experimentally observed position for the proton as being due essentially to environmental effects.Atomic difference maps are presented for the charge density distributions, which reflect the nature of the bonding as it depends on the proton position. Mulliken populations are examined particularly in relation to the “intermolecular” transfer of electrons.
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  • 62
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    International Journal of Quantum Chemistry 35 (1989), S. 395-407 
    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 MO-LCAO-SCF calculations using an STO-3G basis set were performed to find the most stable conformations of L-serine phosphate and hydrated L-serine phosphate. The most favorable conformation of L-serine phosphate is found to be one where the bond sequence O—C—C—C is trans and P—O—C—C gauche, and a very short hydrogen bond is formed between an oxygen atom of the phosphate group and a hydrogen atom of the ammonium group.For hydrated L-serine phosphate, a bridge-type hydration in which a water molecule links a phosphate oxygen and an ammonium hydrogen displays particularly low energy. In the four-hydrated L-serine phosphate anion, the most favorable conformation is such a bridged one having a rather extended configuration with regard to the bond sequences O—C—C—C and P—O—C—C.
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  • 63
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    International Journal of Quantum Chemistry 35 (1989), S. 409-423 
    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 standard formalism of the perturbation theory for time-dependent perturbations follows from the assumption that the exact solution of the unperturbed time-independent problem is known. The consequences of this assumption are analyzed.Particular attention is given to the methods used for the evaluation of transition intensities. It is shown that the traditional intensity formula in most cases violates the assumptions underlying its derivation. It is shown that for the given ansatz concerning the form of the approximate wave function, the transition intensities should be determined from the variation treatment of the corresponding time-dependent problem. The wave functions of both states involved in the transition should be given the same amount of variation flexibility. This condition is satisfied if the transition intensity expression follows from the complete timedependent perturbation treatment of the perturbed system.
    Type of Medium: Electronic Resource
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  • 64
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 441-456 
    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: Previous discussions of the bases and projection operators for projective representations are analyzed and alternatives are proposed. Detailed proofs are provided for a result which is often assumed or proved within unacceptable limitations, namely that the form of the projection operators which is standard for vector representations is also valid for unitary projective representations. These proofs provide necessary conditions for this result, and they are constructed in terms of the definition given for the bases of projective representations. The calculation of Clebsch-Gordan coefficients for projective representations by means of the projection operators is discussed. Whereas in the method of Dirl the work is carried out entirely in terms of the matrix representations, and the symmetrization of the bases has to be considered in a second step, all the work of this paper is conducted starting from the symmetrization of the bases of the projective representations, so that those two steps are carried out simultaneously.
    Type of Medium: Electronic Resource
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  • 65
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    International Journal of Quantum Chemistry 35 (1989), S. 457-457 
    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
    Type of Medium: Electronic Resource
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  • 66
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 459-459 
    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
    Type of Medium: Electronic Resource
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  • 67
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989) 
    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
    Type of Medium: Electronic Resource
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  • 68
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 629-647 
    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: Two approaches in treating the weak representability problem for density operators are studied. In the first approach this problem is reduced to the minimization of a certain distance function. The second approach is based on inductive generation of linear inequalities for diagonal elements of the second-order density operators. Generalization of Kuhn-Yoseloff's results is also discussed.
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  • 69
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 649-663 
    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 explicit relation is derived between the one-body potential energy and the electron density for the ground state of the Be atom in a nonrelativistic framework. This same relation applies to any four-electron atomic ion (or to Be itself) in a state where the electrons occupy two doubly filled orbitals. The relation is interpreted as an exact Hartree-like model of the Hohenberg-Kohn theorem within the general context of N electrons and a potential that is not necessarily spherically symmetrical.
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  • 70
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 701-701 
    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
    Type of Medium: Electronic Resource
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  • 71
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 35 (1989), S. 705-707 
    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
    Type of Medium: Electronic Resource
    Library Location Call Number Volume/Issue/Year