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  • 101
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    International Journal of Quantum Chemistry 38 (1990), S. 97-106 
    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 we propose a new topology, based on the stable attractors of the gradient field of the potential energy hypersurface, that may be used as a general definition of a stable chemical structure. We further construct two examples of particular realizations of this abstract topology. One of them is closely related to the catchment-region topology of Mezey and the other one to the molecular-graph model of Bader. A particularly interesting point is that both topologies lead to equivalent definitions of molecular structure; in particular, they permit us to differentiate between cis-trans isomers and rotational conformers, that had equivalent structures in the definition proposed by Bader.
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  • 102
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    International Journal of Quantum Chemistry 38 (1990), S. 107-117 
    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 crystal orbital calculations on polymers and model molecular orbital calculations on substituted butanes and pentanes, have been carried out to determine the core electron binding energies in fluorinated polyethylenes. The validity of using model compounds and the choice of basis set are critically discussed. Comparable calculations are reported for fluorine substituted methanes and compared with the experimental results. It is concluded that calculations on model compounds, that are more economic than full crystal orbital calculations, can only reproduce the broad trends and cannot give results in agreement with full polymer results.
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  • 103
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    International Journal of Quantum Chemistry 38 (1990), S. 153-166 
    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 pair-excitation-multiconfigurational-self-consistent-field (PEMCSCF) study with 10 configurations has been carried out on 3-methyl-1,2,4-oxadiazolin-5-one (3-methyl pseudosydnone) which has normal covalent bonds but is isoelectronic with 3-methyl sydnone for which no single covalent valence bond structure can be drawn. The PEMCSCF results were found to be very similar for the two isoelectronic compounds and an attempt was made to represent the bonding of the sydnone in only two simple molecular-orbital-valence-bond (MOVB) configurations using a Luken-type correlating orbital. A compact minimum basis of Slater-Transform-Preuss functions fitted by six cartesian gaussian orbitals (STP-6G) is used to express the compact representation for chemical comparison to experiment. The molecular geometry was gradient-optimized at the single determinantal level using a 6-31G extended basis set. “Orthonormal Resonance Analysis” is given for the two dominant configurations by noting that the Boys-Reeves configuration interaction algorithm is identical to the Pauling valence bond algorithm except for use of orthogonal molecular orbitals instead of atomic hybrid orbitals. This equivalence permits use of PEMCSCF atomic charges, bond orders, and dipole moments in a slightly modified valence bond interpretation of the unusual bonding of this compound. Although forcing a larger PEMCSCF problem into a minimum basis and only two configurations raises the energy, chemical interpretation is simplified. This comparison between a mesoionic compound and an isoelectric compound with normal bonding offers new understanding of the mesoionic effect.
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  • 104
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    International Journal of Quantum Chemistry 38 (1990), 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: Density functional calculations - with a local density approximation for the exchange and correlation energy - provide a numerically efficient method for determining total energy variations in molecules. Coupled with molecular dynamics techniques such as simulated annealing, they provide a scheme for probing large regions of configuration space in molecules. We outline the main features of the method and apply it to different ring isomers of S7O, S8, S7Se, Se8, and O8.
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  • 105
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    International Journal of Quantum Chemistry 38 (1990), S. 607-619 
    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 use of finite clusters of atoms to represent the physically interesting portion of a condensed matter system has been an accepted technique for the past two decades. Physical systems have been studied in this way using both density functional and Hartree-Fock methodologies, as well as a variety of empirical or semiempirical techniques. In this article, the author concentrates on the Hartree-Fock based methods. The attempt here is to construct a theoretical basis for the inclusion of correlation corrections in such an approach, as well as a strategy by which the limits of a finite cluster may be transcended in such a study. The initial appeal will be to a modeling approach, but methods to convert the model to a self-contained theory will be described. It will be seen for the case of diffusion of large ions in solids that such an approach is quite useful. A further study of the case of adsorption of rare gas atoms on simple metals will demonstrate the value of inclusion of electron correlation.
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  • 106
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    International Journal of Quantum Chemistry 38 (1990), S. 655-664 
    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 algebraic method is developed for the study of translation vibration energy transfer in a collinear collision between an atom and an anharmonic oscillator. Approximations to the Transition probabilities are obtained with the use of Lie algebraic methods as a function of the collision energy and also as a function of the strength of the anharmonicity.
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  • 107
    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 experimental techniques combined with computer molecular dynamics simulations of increasing complexity are beginning to permit a detailed examination of the initial steps in the complex chemistry describing the high-temperature, high-pressure regime associated with combustion and detonation as well as processes that may be involved in shock-induced chemistry. We present the results of several simulations that describe the generation of moderately strong shocks and their interaction with molecules embedded in weakly-bound clusters, with material defects such as microscopic voids, and with molecular species placed in the neighborhood of such defects.
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  • 108
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    International Journal of Quantum Chemistry 38 (1990), S. 311-320 
    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 improvements in experimental techniques such as inverse photoemission, scanning tunneling microscopy have led to a wealth of new information about crystal structure, electronic states, and dynamical processes at metal surfaces. The level of sophistication reached on the experimental front is yet unmatched by theory and it brings to focus a basic limitation of local density approximation, it gives rise to an exchange and correlation potential vxc(→) with an incorrect asymptotic behavior, the correct asymptotic behavior of vxc(→) is that it should be image-like. The aim of the present article is to investigate what is the effect of introducing an ad hoc asymptotic image behavior (AIB) into the exchange and correlation potential. We are particularly interested in knowing whether image plane position (zo) is inside or outside jellium edge, if it depends on film thickness; as well as knowing if critical thicknesses (Lj) are affected by AIB of exchange and correlation potential.Self-consistent numerical results were derived for the electron density characterizing the bulk Be value (rs = 1.88) in order to allow a direct comparison with previous results [9].
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  • 109
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    International Journal of Quantum Chemistry 38 (1990), S. 383-391 
    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 frequency-independent Breit interaction, which gives the leading correction to the instantaneous Coulomb interaction, is treated self-consistently in matrix Dirac-Fock-Breit self-consistent field (SCF) calculations. Calculations were done on He, He-like ions, Be, Be-like ions, Ne, and Ar. Basis sets of even- and well-tempered Gaussian functions were used to expand the large and small components of Dirac 4-spinors. The results of the variational calculations are compared with Desclaux's benchmark numerical perturbative calculations.
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  • 110
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    International Journal of Quantum Chemistry 38 (1990), S. 419-427 
    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: Preliminary results from our charge self-consistent LCAO band structure (CSCBS) calculations with Bloch sums as the basis reveal that a noncorrosive reduced band gap electrode for photoelectrochemical solar cells may be produced from a (1:1) mixture of β-PbO2 and TiO2 (both rutile). The band gaps for the constituents (β-PbO2 and TiO2) and the 1:1 mixture are calculated and a detailed characterization of the valence and the conduction bands is undertaken to offer a possible mechanism for the reduction of the band gap of the mixture. The band gap for the perovskite PbTiO3 is also calculated to offer a guideline for selecting from the competing pathways to the fabrication of noncorrosive photoelectrochemical electrodes.
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  • 111
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    International Journal of Quantum Chemistry 38 (1990), S. 481-485 
    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 have derived exact analytical expressions for the diagonal matrix elements 〈n|xk|n〉, k = 1, 2, for the Morse oscillator using the operator algebra of the two-dimensional harmonic oscillator and the method of the parameter differentiation. These expressions avoid the roundoff error present in other relations. The method can be extended to other integrals where k 〉 2.
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  • 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: Molecular dipole oscillator strength distributions (DOSDS), which are reliable in the sense of yielding accurate results for the various dipole properties of molecules, can be constructed if sufficient input information is available. The DOSDS are constructed from extensive experimental and theoretical information, including oscillator strength and photoabsorption cross section data, and are constrained to reproduce accurate refractivity measurements of the relevant dilute gases and to satisfy oscillator strength sum rules. The techniques for obtaining the DOSDS, and related properties, are well established for the evaluation of reliable isotropic dipolar dispersion energies. In this paper they are extended to the evaluation of anisotropic dipole properties, with particular emphasis on anisotropic dispersion energies. The approach used will be illustrated with applications involving H2, N2, and the rare gases. Also included will be brief discussions on (i) the use of reliable results for the dipole-dipole and triple-dipole dispersion energies in constructing complete two- and many-body potential energies and (ii) the advantages of constrained DOSD methods, relative to other approaches, for determining all the dipole properties of molecules.
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  • 113
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    International Journal of Quantum Chemistry 38 (1990), S. 563-567 
    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 results of selfconsistent, first-principles calculations of total energies for AsGa and SbGa in GaAs. We confirm that both impurities in the substitutional Td site behave as double donors, and the first internal excitation appears at around 1 eV. For the neutral systems we obtain a metastable minimum in the total energy surface in a configuration with the impurity atom displaced toward the interstitial site; the transformation to this metastable configuration, however, is not expected to be operative for the SbGa defect.
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  • 114
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    International Journal of Quantum Chemistry 38 (1990), S. 569-584 
    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 many-body potential of density-functional theory is thus far understood in its mathematical context as the functional derivative of the exchange-correlation energy functional of the density. In recent work we have attempted to provide a physical interpretation for this potential. We interpret it as the work required to move an electron against the electric field of its Fermi-Coulomb hole charge distribution. Implicit in this interpretation is that the potential is path-independent. For symmetric systems this is rigorously the case. For systems where this may not be the case, the potential may be derived from an effective charge distribution given by the divergence of the field, thus ensuring its path independence. Also implicit as a consequence of the total Coulomb hole charge being zero is that the asymptotic structure of the potential is entirely due to the Fermi hole charge distribution, and thus known precisely. The potential lies explicitly within the rubric of density-functional theory in that within the exchange-only approximation it satisfies the exchange energy virial theorem sum rule and all scaling properties that the exact exchange potential must satisfy. The potential does not satisfy the virial theorem sum rule for the correlation energy, and consequently a term proportional to the difference between the interacting and noninteracting system kinetic energies must be added for the sum rule to be satisfied exactly. The formalism differs from density-functional theory in that it is not derived from the variational principle for the energy, thus obviating the requirement of determining functional derivatives, as well as allowing for the study of excited states. The interpretation also leads to insights into the exact Slater exchange potential, and other approximation schemes such as the Xα method, and local density and gradient expansion approximations. The results of application to few-electron atomic and many-electron metallic surface inhomogeneous electronic systems are remarkably accurate when compared with other theoretical calculations and experiment.
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  • 115
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    International Journal of Quantum Chemistry 39 (1991), S. 173-181 
    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 original procedure has been developed to determine the eigenvalues and the eigenfunctions of quantum-mechanical many-body systems with a harmonic oscillator interaction. Exact solutions in the explicit form have been obtained for the case of systems containing three groups of identical particles. For the system containing two groups of identical particles, a solution in the adiabatic approximation has been obtained and compared with the exact one.
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  • 116
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    International Journal of Quantum Chemistry 39 (1991), S. 229-229 
    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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  • 117
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    International Journal of Quantum Chemistry 39 (1991), S. 233-233 
    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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  • 118
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    International Journal of Quantum Chemistry 39 (1991), S. 331-344 
    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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  • 119
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    International Journal of Quantum Chemistry 37 (1990), S. 257-269 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Accurate lower and upper bounds for the nonrelativistic ground state energies E0 of the real systems 4He, H-, D-, and T- were calculated by the method of variance minimization using wave functions which include logarithmic terms. In addition, an analogous treatment with an infinite mass approximation for H-, He, and the isoelectronic series up to Z = 10 was carried out. Especially for H- and He the results (a.u.) are given by \documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} { - 0.52775101712297_4 〈 E_0 (H^ -) 〈 - 0.52775101654373_9 } \\ { - 2.90372437703413_4 〈 E_0 (He) 〈 - 2.90372437703411_9 .} \\ \end{array} $$\end{document} These values have an absolute error smaller than 1.28 · 10-4 cm-1 for H-1and 3.10 · 10-9 cm-1 for He. Moreover it is shown that a variation of nuclear mass mn for the H- species does not produce a second discrete eigenvalue.
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  • 120
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    International Journal of Quantum Chemistry 37 (1990), S. 311-311 
    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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  • 121
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    International Journal of Quantum Chemistry 37 (1990), S. 319-325 
    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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  • 122
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    International Journal of Quantum Chemistry 37 (1990), S. 327-347 
    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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  • 123
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    International Journal of Quantum Chemistry 37 (1990), S. 403-414 
    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 LCAO method is used for a discussion of the π electron approximation. The Hückel method is presented and rephrased on the basis of Löwdin orthogonalized orbitals. The background of the Pariser-Parr-Pople model Hamiltonian is illuminated and the method is derived from first principles. The parametrization is discussed and the scope reviewed.
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  • 124
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    International Journal of Quantum Chemistry 38 (1990), S. 767-780 
    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 classical mechanics, the equations of motion for conservative systems, i.e., systems in which the sum of the kinetic energy T and potential energy V is constant, can be derived from a Lagrangian function \documentclass{article}\pagestyle{empty}\begin{document}$ L(\dot q,q) = T(\dot q) - V(q) $\end{document} via the Euler-Lagrange equation \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {{d \mathord{\left/ {\vphantom {d {dt}}} \right. \kern-\nulldelimiterspace} {dt}}} \right)\left( {{{\partial L} \mathord{\left/ {\vphantom {{\partial L} {\partial \dot q}}} \right. \kern-\nulldelimiterspace} {\partial \dot q}}} \right) - {{\partial L} \mathord{\left/ {\vphantom {{\partial L} {\partial q}}} \right. \kern-\nulldelimiterspace} {\partial q}} = 0 $\end{document}, or from the Hamiltonian function H(p, q) = T(p) + V(q) via the Hamiltonian equations \documentclass{article}\pagestyle{empty}\begin{document}$ \dot q = {{\partial H} \mathord{\left/ {\vphantom {{\partial H} {\partial p}}} \right. \kern-\nulldelimiterspace} {\partial p}},\dot p = {{ - \partial H} \mathord{\left/ {\vphantom {{ - \partial H} {\partial q}}} \right. \kern-\nulldelimiterspace} {\partial q}} $\end{document}. The Hamiltonian function represents the energy of the system in terms of generalized coordinates q and momenta p, and is used as a basis for the so-called canonical quantization to obtain the corresponding Hamiltonian operator and, thus, the Schrödinger equation for the system. For dissipative systems, i.e., systems where mechanical energy is lost due to frictional forces, difficulties arise in the analogous for formalism even on the classical level. Although formal Lagrangian functions yielding the correct equations of motion can be given, problems exist in the physical interpretation of these Lagrangians and the corresponding Hamiltonians. This becomes even more obvious when these Hamiltonians are quantized in the usual canonical way. A major problem is connected with the violation of the uncertainty principle. A new classical Lagrange-Hamilton formalism will be proposed, which not only yields the correct equations of motion for position and momentum, but also allows a physical interpretation of the Hamilton function in terms of the energy of the system. Following the canonical method, the time derivative of physical quantities can be obtained via a modified Poisson bracket formalism. The question of the quantization of such classical dissipative systems is discussed using the most important examples, the damped harmonic oscillator, and the damped free particle. Special attention is paid to the problem of vanishing uncertainty products.
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  • 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: We have recorded and analyzed the HeI PE spectra of the following molecules: 9,10-dihydroanthracene (1); 9,10(H)-anthracenone (2); 9,10-anthracenedione (3); 1,2,3,4-tetrahydro-9,10-anthracenedione (4); 1,4,1a,4a-tetrahydro 9,10-anthracenedione (5); 10-methylene-9,10(H)-anthracenone (6); and 10-(phenylmethylene)-9,10(H)-anthracenone (7). The PE spectra are assigned by comparison with those of the composite parts (i.e., by employing an orbital interaction model such as the composite molecule method). This approach, which works surprisingly well in the present instance, indicates that the carbonyl lone pair and the carbonyl π electrons interact negligibly with the outer π electrons of the aromatic unit(s). If no change in conformation of the component aromatic parts occurs, the spectrum of the composite molecule exhibits the additivity property. This result agrees with previous studies of benzophenones [3]. However, it is argued that the ordering of the strongly overlapped, low energy ionization bands of benzaldehyde and acetophenone should be changed to I1(π), I2(π) ≍ I3(no) and I1(π), I2(no), I3(π), respectively.
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  • 126
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    International Journal of Quantum Chemistry 38 (1990), S. 859-871 
    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 find the best methods for calculating the (hyper) polarizabilities of long oligomers and infinite polymers, we first propose in this paper to calculate the linear static electric polarizabilities of regular and alternant polyene chains by using the Hückel approach. For these calculations, the perturbation approach is used to obtain second-order response properties. We have derived this formulation by starting from the polarization propagator within an approximation which consists of projecting the superoperator resolvent form on a basis set of operators composed of only single excitations. For polyenes of different degrees of bond alternation, we obtained in the same Hückel approximation the asymptotic value of the polarizability per unit cell by using the Bloch functions in the Genkin and Mednis technique of calculating polymeric polarizabilities.
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  • 127
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    International Journal of Quantum Chemistry 39 (1991), S. 1-2 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 128
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    International Journal of Quantum Chemistry 39 (1991), S. 93-113 
    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 excitation and ionization spectra of RuO4 and OsO4 are studied theoretically by the symmetry-adapted cluster (SAC) and SAC-CI theories. This is the attempt to assign whole of the spectra by ab initio calculations including electron correlations. In the ground state, electron correlations work to reduce the polarity of the M-O bond overestimated in the Hartree-Fock calculation. The Os-O bond is stronger than is the Ru-O bond, which is reflected in the differences of the excitation and ionization spectra of RuO4 and OsO4. The excitation energies of the experimental spectra are well reproduced by the SAC-CI theory, though the calculated intensities of some peaks are very small in comparison with the experiments. The outer-valence ionization spectra calculated by the SAC-CI theory agree well with the experimental photoelectron spectra. Some shake-up peaks that are accompanied with an electron-transfers from oxygen to metal are also calculated.
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  • 129
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    International Journal of Quantum Chemistry 39 (1991), S. 159-162 
    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 dependence of the rainbow angle on the depth of the potential well, the equilibrium distance, andthe force constant of the potential are discussed for several potential functions.
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    International Journal of Quantum Chemistry 39 (1991), S. 183-196 
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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 exact, within the Born-Oppenheimer approximation, body-fixed Hamiltonian for the nuclear motions of a triatomic system is presented. This Hamiltonian is expressed in terms of two arbitrarily defined internal distances and the angle between them. The body-fixed axis system is related to these coordinates in a general fashion. Problems with singularities and the domain of the Hamiltonian are discussed using specific examples of axis embedding. A number of commonly used coordinate systems including Jacobi, bond-length-bond-angle, and Radau coordinates are special cases of this Hamiltonian. Sample calculations on the H2S molecule are presented using all these and other coordinate systems. The possibility of using this Hamiltonian for reactive scattering calculations is also discussed.
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    International Journal of Quantum Chemistry 40 (1991), S. 1-2 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 40 (1991), S. 33-53 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: A behavior of a quantum rigid and pointlike dipole in a permanent electric field is under study. A dependence of a number of librational energy levels on the strength of the electric field is investigated on the basis of some rigorous results for the appropriate normal Jacobian matrices.
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    International Journal of Quantum Chemistry 40 (1991), S. 139-148 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: A very simple modification of the usual (∼N4) SCF procedure is proposed, permitting the exclusion of basis set superposition errors (BSSE) in problems of intermolecular interactions. No a posteriori corrections are required. The results of this “CHA/F method” are numerically close to those of the Boys-Bernardi correction scheme but are free from the “overcompensation” characteristic of the latter at smaller distances.
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    International Journal of Quantum Chemistry 40 (1991), S. 243-264 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The singly excited functions satisfying Brillouin theorem are expressed as linear combinations of configuration-state functions for any spin and spatial symmetries (atomic or molecular) and for any reference wave function. The generality of the formulation is ensured by the use of the irreducible tensor method that can be adapted to any symmetry point group of interest. The expansion coefficients are simply written as products of fractional parentage coefficients, spin- and orbit-recoupling coefficients, and phase factors. The formalism is illustrated for some atomic (Kh) and molecular (C∞v, C3v, and Td) configurations. Group theoretical techniques are also used to correlate the Brillouin conditions within a chain of groups.
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    International Journal of Quantum Chemistry 40 (1991), S. 299-304 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: A model for catecholamine storage in vesicles is analyzed within the SCRF-CNDO/2 approach including continuum solvent effects. The model considers the approach of cationic norepinephrine (NE) to a positively charged guanidinium moiety. Ion-pair formation is found for the whole range of dielectric constants. Even though stable states of H-bonded partners are found for large dielectric constants, this process is ruled out to occur because it involves too high energies. It appears that the medium's polarity is determinant in lowering the energy barrier between the ion-pair complex and the separated partners. Thus, as the medium dielectric constant increases, the equilibrium between the two states is enhanced.
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    International Journal of Quantum Chemistry 39 (1991), S. 519-519 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 39 (1991), S. 487-517 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: We present a general method of implementing the kinetic balance condition within the Dirac-Fock (DF) self-consistent field (SCF) formalism for closed-shell molecular structure. We review the steps leading to the derivation of DF SCF equations for closed-shell molecules, particularly as formulated by Matsuoka et al. In the present approach, the large component of the molecular spinors are expanded in terms of atomic basis spinors of spherical-type Gaussian functions, with the small component related to the large component by the kinetic balance condition. It is shown that imposing the kinetic balance condition on geometric Gaussian-type basis functions allows us to obtain the Fock matrix elements, involving both the large and the small components, form the standard nonrelativistic Cartesian-type matrix elements. By using properties of orthogonal polynomials, the solid spherical harmonics are expressed in Cartesian form, thus providing a general basis for transformation of one- and two-electron-matrix elements, obtained from a Cartesian Gaussian-type basis, to a spherical Gaussian-type basis. The advantages of using kinetically balanced geometric Gaussian-type basis functions in molecular DF calculations including finite-size nucleus effects are emphasized. For the sake of completeness, we have added in an appendix corrections to the nuclear attraction matrix elements for the finite-size nucleus already derived by Matsuoka.
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    Notes: Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations, based on localized/local orbitals and an “effective” CI Hamiltonian, for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystalline or other solid environment.Our technique begins with an explicit quantum chemical SCF calculation for a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized, and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are then carried out for breaking a bond in the reference molecule. This method is completely general in that the space treated explicitly, as well as the surrounding space, may contain voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc.Dimethylnitramine is the smallest prototype of the energetic R2N - NO2 nitramines, such as the 6-member ring RDX or the 8-member ring HMX. Decomposition of energetic compounds is initiated in the solid by a breaking of the target bond. Thus, it is crucial to know the difference in energy between breaking a bond in an isolated energetic molecule versus in the molecule in a solid. In the present study, we have carried out MRD-CI calculations for the Me2N - NO2 dissociation of dimethylnitramine in a dimethylnitramine crystal. The cases we investigated were one dimethylnitramine molecule (surrounded by 53 and 685 neighboring dimethylnitramine molecules represented by multipoles), three dimethylnitramine molecules, and three dimethylnitramine molecules (surrounded by 683 neighbors). All multipoles were cumulative atomic multipoles up through quadrupoles. The MRD-CI calculations on dimethylnitramine required large numbers of reference configurations from which were allowed all single and double excitations.
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    International Journal of Quantum Chemistry 39 (1991), S. 667-679 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The frequency-dependent dipole polarizability, α(E), is calculated using the second-order polarization propagator approximation (SOPPA). We have shown how to express α(E) as a function of E2 and thus obtained a form of α(E) that can be used to compute C6-coefficients without invoking complex arithmetic. For He we find that SOPPA recovers a large fraction of the correlation contribution for all frequencies, whereas for H2 where the correlation contributions are much smaller and also basis set-dependent, we find a less definite trend of SOPPA relative to RPA.
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    International Journal of Quantum Chemistry 37 (1990), S. 121-123 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Each undirected graph has its own adjacency matrix, which is real and symmetric. The negative of the adjacency matrix, also real and symmetric, is a well-defined mathematically elementary concept. By this negative adjacency matrix, the negative of a graph can be defined. Then an orthogonal transformation can be readily found that transforms a negative of an alternant graph to that alternant graph: (-G) → G. Since the procedure does not involve the edge weights, the pairing theorem holds true for all edge-weighted alternant graphs, including the usual “standard” graphs.
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    International Journal of Quantum Chemistry 37 (1990), S. 51-68 
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    Topics: Chemistry and Pharmacology
    Notes: For the general two-electron two-center integral over Slater functions, use of the Neumann expansion for the electron-electron interaction term yields the standard auxiliary functions. These are expanded and integrated explicitly by two independent methods. The resulting simple analytic formula for the total integral is completely general, requiring only the Slater function quantum numbers and exponents and the internuclear separation. Hence all two-electron hydrid, coulomb, exchange, and one-center integrals are considered. The efficiency of calculation of this expression is compared with those of other methods, indicating an order of magnitude improvement in speed over recursion for the exchange integral.
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    International Journal of Quantum Chemistry 37 (1990), S. 423-436 
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    Topics: Chemistry and Pharmacology
    Notes: Symmetries are considered for a class of Hamiltonian models with one (spin-free) orbital per site. The models include common types of Parisier-Parr-Pople and valence-bond Hamiltonians, defined over a continuous range of parametrizations. The symmetries investigated are linear canonical transformations and include the common point-group and alternancy symmetries. We find “graphical” symmetries extending the usual point-group symmetries and novel “color-pairing” symmetries which involve hybrids of point-group-like and alternancy symmetries of relevance for certain heteroatomic species. The occurence of recolorpairing transformations relating the eigensolutions of models for different molecules is also noted.
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    International Journal of Quantum Chemistry 37 (1990), S. 113-120 
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    Topics: Chemistry and Pharmacology
    Notes: McClelland's rules on graph splitting can be represented using the generalized graph notation. Generalized graphs are edge- and vertex-weighted graphs, which are becoming important to chemical problems. By this the McClelland method of graph splitting has a wider range of applications. “Stack graphs” are constructed from identical “base graphs” by connecting corresponding vertices from one base to another. Their eigenvalues are related to the eigenvalues of the base graph. Two- and even three-layered graphs may be used as a simple model for the inter-ring interaction in a cyclophane.
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    International Journal of Quantum Chemistry 37 (1990), S. 85-94 
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    Topics: Chemistry and Pharmacology
    Notes: Scalar nonlinearities are operators of the form N(u) = 〈Au, u〉Bu, where A, B are linear operators, and 〈·,·〉 is the inner product in a Hilbert space H. This paper reviews applications of scalar nonlinearities. We show that operators of the form N(u) are found in equations that describe phenomena of classical mechanics, open systems of quantum mechanics, and chemical physics.
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    International Journal of Quantum Chemistry 37 (1990), S. 797-809 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The matrix elements of various analytical functions f(X), X being the internuclear separation, are required for the description of transition probabilities and other molecular properties. These matrix elements can be conveniently estimated by assuming vibrational wave functions of two relatively dispalced linear harmonic oscillators of arbitrary frequencies to represent the vibrational levels of two electronic states of a molecule. Using this assumption, analytical expressions for the matrix elements of an arbitrary analytical function f(X) are obtained. Useful recursion relations among these matrix elements are derived and an elegant graphical representation of the recursion relations is obtained. These graphical representations are utilized to obtain new more general recursion relations among matrix elements of the arbitrary function f(X).
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    International Journal of Quantum Chemistry 37 (1990), S. 821-821 
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    International Journal of Quantum Chemistry 37 (1990), S. 465-483 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: PPP MO theory initiated in 1953 a reliable SCF approach to excited states in multielectronic systems. Further developments, such as Del Bene-Jaffé's CNDO/S method, modified Pople's CNDO all-valence electron (AVE) approach to improve the results but maintained some basic troubles of this theory which were not in the PPP method. A new approach to SCF-MO-NDO calculations with a priori chosen parameters and configuration interaction of singly excited states is developed in this paper from the ground considerations of the PPP method and it is called CNDOL. Thus, AVE NDO approximations have been modified in order to take into account independent monocentric terms in the Fock matrix elements for each kind of azimuthal quantum number atomic orbitals in the basis set. The only input data are Slater exponents, valence state ionization potentials, and electron affinities of each kind of atomic orbital. No parameters are empirically adjusted to fit results to experimental or previous accurate theoretical data. CNDOL is suitable for obtaining theoretically consistent results in polyatomic systems which are too large to be treated by more sophisticated procedures. Applications to hydrocarbons are shown which compare PPP results with the new AVE approach derived from it here.
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    International Journal of Quantum Chemistry 37 (1990), S. 517-528 
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    Topics: Chemistry and Pharmacology
    Notes: A comparison between some different modifications of PPP methods is made. It is noticed that each modification may have its advantages but that some may be more suitable for interpretation of electronic spectra than others. It is also pointed out that PPP methods may allow more accurate descriptions of π properties than all-valence methods, such as CNDO/S.The Peel method, a PPP method especially adapted to reproduce the lowest π → π* transitions of a few reference molecules, has been applied to studies of the pyrimidine bases cytosine, uracil, and thymine. The obtained frequencies and oscillator strengths compare favorably with experimental evidence. The calculated polarization directions are in general agrreement with measurements. In case of cytosine a reinterpretation of measurements is suggested.
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    International Journal of Quantum Chemistry 37 (1990), S. 529-537 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The localization properties of the π-electron system of some aromatic and conjugated hydrocarbon molecules have been investigated. The π orbitals have been obtained using a standard PPP-SCF procedure. The shape analysis was carried out using the novel concept of the relation between the localization measure and structural entropy. According to our investigations the π-electron system of these aromatic molecules is more localized than expected. The eigenstates seem to show one-dimensional characteristics with a decay rate generally faster than exponential. Linear chains, on the other hand, have rather delocalized orbitals. As a byproduct we have found that in certain systems some states exhibit step-function-like behavior, similar to bond-centered charge density waves.
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    International Journal of Quantum Chemistry 37 (1990), S. 539-546 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: By making the atomic effective charge self-consistent with the molecular charge density, it is shown that the PPP method can accurately predict the ionization potential and electron affinity computed as an energy difference of separately minimized ground and ionized states. The calculation is applied to a large variety of conjugated organic molecules, including heterocyclic systems and π-electron-contributing substituent groups.
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    International Journal of Quantum Chemistry 37 (1990), S. 609-609 
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    International Journal of Quantum Chemistry 37 (1990), S. 631-653 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Subduction coefficients adapted to the group chain, which appeared in Racah's treatment of fn configurations, are defined and calculated in the unitary group approach. The coefficients are then utilized to construct successively adapted term functions and evaluate other interesting coefficients. In addition the simplified expressions for the Coulomb and spin-orbit operators are obtained in terms of generators.
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    International Journal of Quantum Chemistry 37 (1990), S. 679-683 
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    Topics: Chemistry and Pharmacology
    Notes: In the present paper, the band structures of valence-band and conduction-band for polyacetylene and Lidoped polyacetylene were calculated by LCAO-SCF ab initio crystal orbital (CO) method. The influences of doping on the geometric structures, the band structures, and the changes of atomic charges were analyzed and discussed to explain the insulator-conductor transition and electric conducting mechanism of doped polyacetylene.
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    Notes: The possible use of a half-projected Hartree-Fock (HPHF) wave function correlating a single pair of electrons, being defined as a linear combination of two unrestricted Slater determinants which are identical except that all the α and β spin functions are interchanged, is investigated in relation to the computation of the potential energy surfaces involving singlet biradical or biradical-like species. The results of calculations, within the framework of the MINDO/3 semiempirical Hamiltonian, for a number of prototypical biradical molecules indicate that, in general, the HPHF model provides a geometrical and energetical description that is equivalent to that obtained from a conventional two-configuration self-consistent-field wave function.
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    International Journal of Quantum Chemistry 38 (1990), S. 11-23 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Electrostatic potential maps of benz(a)anthracene diol epoxide, benzphenanthrene diol epoxide, chrysene diol epoxide, and tryphenylene diol epoxide, which are ultimate carcinogens derived from the corresponding polycyclic aromatic hydrocarbons, have been studied using a dipole of length 1 Å and strength 1 Debye. The net charge distributions in the molecules were obtained using the MNDO molecular orbital method. The maps were drawn for two closest distances of approach between the charged ends of the dipole and atoms of the molecules. Using the electric field directions and values of electrostatic potentials, reactive sites and relative reactivities of these molecules have been examined. Existence of a bay region appears to be important for the carcinogenic activity of a diol epoxide in this class of carcinogens.
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    International Journal of Quantum Chemistry 38 (1990), S. 97-97 
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    International Journal of Quantum Chemistry 38 (1990), S. 121-128 
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    Topics: Chemistry and Pharmacology
    Notes: (SiC)2 clusters occupy an intermediate place between the well-studied clusters Cn and Sin. Of the five closed-shell isomers investigated in this paper, the rhombic form is found to be the lowest in energy at the MBPT(4) level. Analytic SCF and MBPT(2) second derivatives are used to predict vibrational frequencies for the isomers.
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    International Journal of Quantum Chemistry 38 (1990), S. 447-460 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: In the generalized Hylleraas-CI method, the original correlation factor rvij is multiplied by a Gaussian geminal. Using the approach of generating functions, the general formulas of molecular integrals in this method are derived over Cartesian Gaussian orbitals. From differentiations of the generating functions, the expanding length in the incomplete Gamma functions is reduced, and some cancellations presented in other approaches are avoided. Preliminary calculations for H2 and H2 - H2 systems are carried out over STO-3G basis. The results are encouraging.
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    International Journal of Quantum Chemistry 38 (1990), S. 119-120 
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    International Journal of Quantum Chemistry 38 (1990), S. 139-147 
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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: Diagrammatic formulation of the MBPT is applied when the occupied and the virtual canonical orbitals are separately localized by unitary transformations. In this localized representation, due to the off-diagonal Fock matrix elements, the perturbation operator contains extra terms generating the so-called localization corrections. These corrections enter the perturbation energy in third and higher orders. Their magnitude depends on the type of localization, but they represent only a small fraction of the canonical corrections. The calculation of the localization corrections, however, does not need a significant amount of extra computer time. It is shown that by introducing an “order of neighborhood” local and nonlocal effects of the electron correlation can be separated and the contribution of the nonlocal effects can be neglected to a good approximation. Ab initio calculations have been carried out for the normal saturated hydrocarbons: C2n+1H4n+4 and for the all-trans conjugated polyenes C2n+2H2n+4. As to the ratio of the local and nonlocal corrections, it is shown that there is only a quantitative difference for these two kinds of systems (strongly or weakly localizable). Neglecting nonlocal effects, considerable amount of computer time can be saved.
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  • 165
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    International Journal of Quantum Chemistry 38 (1990), S. 191-208 
    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 scheme for systematic reduction of the theoretical treatment of elementary reactions involving polyatomic molecules is described; it consists of (1) limitation to the energetically relevant regions of the nuclear configuration space (the reaction path and its near environs) and (2) restriction to the dynamically relevant subspace of the nuclear configuration space (the active modes). Starting from a generalized reaction path Hamiltonian of Nauts and Chapuisat allowing for the use of arbitrary curvilinear coordinates and several large-amplitude modes, the realization of the above-sketched scheme is discussed. A compilation of recent work along these lines, mostly based on the simplified Miller-Handy-Adams reaction path Hamiltonian, is given with particular emphasis on applications of a statistical adiabatic model.
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  • 166
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    International Journal of Quantum Chemistry 38 (1990), S. 221-224 
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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 quantum chemical substantiation of antioxidant action of the substituted phenols in polyolefins is presented. The electronic structure of the phenols, phenoxy radicals, and model products of polyolefin destruction was calculated by the MNDO method. On the basis of the presumed chemical reactions, the electronic structure criteria determining the antioxidant activity are selected. These are the O - H bond dissociation energy in phenols and the charge on the atoms of π-system in phenoxy radicals. The competition between these reactions takes place, and the calculation of the reaction heats shows that the reactions with the formation of the substituted quinones are in preference to the reactions in which the peroxycyclo-hexadienones are formed.
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  • 167
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    International Journal of Quantum Chemistry 38 (1990), S. 245-252 
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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: Ab initio SCF calculations were performed to study the conjugation of C, N, Si, and P double bonds with BH2 (π-acceptor) and NH2 (π-donor). The variations of the energy, geometry, and electronic distribution on rotation ZH2 groups connected to the double bonds depend greatly on the polarities and polarizabilities of the molecules under study. The repulsive (attractive) interactions of the lone pairs lying in the plane of the double bond with donor (acceptor) orbital can modify strongly the relative stabilities of the conformations and the parameters of the molecule and electronic structures.
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  • 168
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    Topics: Chemistry and Pharmacology
    Notes: The 2H, 6Li, and 7Li quadrupole coupling constants of the low-lying rovibrational levels in the X1∑+ and A1∑+ electronic states of 7Li2H, 6Li2H, 7Li1H, and 6Li1H are calculated from molecular wave functions which explicitly describe nuclear motion. Except for the lithium coupling constants in the A1∑+ state, the vibrational dependence of the nuclear quadrupole coupling is found to be significant for all the studied isotopic species. On the other hand, the rotational dependence appears to be important only for the deuteron coupling constant of 7Li2H and 6Li2H in the A1∑+ state. Special attention is paid to relative magnitudes of the Li and D quadrupole coupling constants in the 7Li2H and 6Li2H isotopic species. The information about the relative magnitudes and their changes with vibrational excitation is used to outline a theoretical approach to the interpretation of the hyperfine structure of the spectra of these species.
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  • 169
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    International Journal of Quantum Chemistry 38 (1990), S. 329-338 
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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 generalization of the well-known Hückel rule of quantum chemistry is combined with the crossing theorem of the method of moments to interpret the calculated properties of two adatoms on the (001) surfaces of the simple cubic metal and the B.C.C. transition metal. The relation between the fourth-order energy moment and the repulsive interaction between adatoms is emphasized. Some conclusions are drawn on the mechanism of the W (001) surface reconstruction.
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  • 170
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    International Journal of Quantum Chemistry 38 (1990), S. 357-372 
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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 practical procedure for calculation of nuclear quadrupole coupling constants for light diatomic molecules is discussed. The procedure is based on a molecular wave function that explicitly describes nuclear motion. The approach is capable of yielding quadrupole coupling constants for excited rovibrational levels of diatomic molecules in their ground and excited electronic states. An application of the procedure to the X1Σ+g and B1Σ+u states of HD and D2 is presented.
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  • 171
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    International Journal of Quantum Chemistry 38 (1990), S. 699-711 
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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: By combining a well-known theorem of Longuet-Higgins on the phase changes of electronic wave functions along loops encircling a conical intersection of potential surfaces, and a recently proven “vertical symmetry theorem” on the relative distributions of point symmetry groups and potential surface critical points in configuration space, a new result is obtained, suitable for the prediction of the presence of a critical point within a domain of configurations.
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  • 172
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    International Journal of Quantum Chemistry 38 (1990) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 173
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    International Journal of Quantum Chemistry 38 (1990), S. 819-830 
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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 Wigner distribution function and the corresponding density matrix are calculated using a form for the distribution function suggested by maximization of the entropy. Wigner functions and density matrices are determined by imposing conditions of idempotency on the density matrix. Exchange energies and Compton profiles calculated from density matrices obtained by imposing the idempotency constraints are compared with the results of calculations using the Hartree-Fock density matrix and a Gaussian approximation for the density matrix for H and the noble gases He through Xe. Compton profiles from Wigner functions with idempotency constraints show improvements over the Gaussian approximation for the lighter atoms, but do not show significant changes for the heavier atoms. Exchange energies from density matrices with idempotency constraints show improvements over the Gaussian approximation except for the heavier atoms Kr and Xe.
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  • 174
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    International Journal of Quantum Chemistry 38 (1990), S. 161-171 
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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, the distance between the F atom of the substrate 9-fluorocamphor and the iron of the heme unit in cytochrome P450cam was determined by nuclear magnetic resonance (NMR) and found to be 0.8 Å shorter than the value obtained from examination of the camphor bound crystal structure. In an attempt to resolve this disparity, we have used semi-empirical quantum mechanical and empirical methods to reevaluate the X-ray based estimate of the Fe - F distance taking into account the possibility of differences in the binding orientation and motion of camphor and 9-fluorocamphor in the binding site of P450cam. Our results suggest that the difference arises from small changes in the orientation of the two substrates and restricted, as opposed to free, rotation of the C9 methyl group in 9-fluorocamphor. With these factors taken into account, our calculations yield a value of 3.8 Å for 9-fluorocamphor which is the same as determined by NMR. Therefore, our results support the conclusion that the origin of the difference between the NMR and X-ray derived Fe - F distance for 9-F-camphor is a real difference in the binding of the two substrates, as well as hindered rotation of the methyl group in the binding site.
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  • 175
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    International Journal of Quantum Chemistry 38 (1990), S. 015-028 
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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: Properties of linear orbital transformations that leave the first-order density matrix invariant are investigated. These isopycnic orbital transformations make possible to localize not only the Hartree-Fock spin-orbitals, but the natural spin-orbitals, as well. A new localization procedure, based on atomic overlap matrices, is proposed to facilitate interpretation of the localized natural spin-orbitals (LNOS). Changes in LNOS upon breaking of a single bond in the H2 and HF molecules are examined. The LNOS are also computed for the CO and LiF molecules and compared with the localized molecular orbitals.
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  • 176
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    International Journal of Quantum Chemistry 38 (1990), S. 675-689 
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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 molecular cluster model approach for the description of processes at surfaces is presented. It involves using finite clusters that contain surface atoms interacting with atomic or molecular adsorbates. Accurate ab initio wave functions can be obtained for the clusters, and, thus, this makes it possible to describe the bonding in terms that are familiar for molecular systems. The adsorbate-substrate interaction can be analyzed by new techniques developed to characterize the bonding as covalent or ionic. This is shown for F on Ag(111) and O and CO on Cu(100) systems. The measures of adsorbate ionicity are (a) the expectation value of a projection operator giving an indication of the number of electrons associated with the adsorbate, (b) the analysis of the dipole moment curve as function of the distance, and (c) the effect of an external electric field on the adsorbate geometry. The consequences of ionic adsorbates on metal surfaces is discussed for two selected topics: adsorption of thiocyanate on Ag electrodes and coadsorption of CO with ionic species.
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  • 177
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    International Journal of Quantum Chemistry 38 (1990), S. 119-131 
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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 methods of theoretical chemistry were used to characterize the molecular structure and some reactivity properties of (-)-9-nor-9β-hydroxyhexahydrocannabinol (9-nor-9β-OH-HHC), our template molecule for cannabinoid analgesia. This characterization is part of a project whose ultimate goal is the design of cannabinoid analgesics with reduced psychoactive liabilities. Our working hypothesis is that the analgesic activity of 9-nor-9β-OH-HHC is due to the presence and relative location of two regions of negative potential in the top half of the molecule. A complete conformational study of the fused ring structure of 9-nor-9β-OH-HHC was performed using the technique of molecular mechanics as encoded in the MMP2(85) program. This study revealed six accessible conformers of 9-nor-9β-OH-HHC. All six conformers were found to have the same fused ring conformation, but to differ in the position of the phenol and 9β-hydroxyl groups. Molecular electrostatic potential (MEP) maps of each accessible conformer were calculated from molecular wave functions obtained with the LP-3G basis set implemented into the Gaussian 80 program. The MEP maps calculated at distances of 1.5 and 3.3 Å from the molecular plane defined by the aromatic ring serve as a discriminative factor for the conformers of the studied compound. In order to quantiate the spatial relationship of the potential minima in the MEPs of each accessible conformer, points of minimum potential associated with the 9β-hydroxyl oxygen (X1 at -1.5 Å and X2 at -3.3 Å) and with the phenol oxygen (Y1 at 1.5 Å and Y2 at -1.5 Å) were identified in the MEP maps of each conformer. The distances, |;XnYn|, and the nonbonded torsion angles, Yn-O-C9-Xn, were then measured for all conformers. The accessible conformations of 9-nor-9β-OH-HHC along with their MEPs and the measurements |XnYn| and Yn-O-C9-Xn form our preliminary template for cannabinoid analgesia. Future comparisons of this template with the properties of active and inactive cannabinoid analgesics should permit the identification of the relevant conformer at the site of action of these compounds and permit the formulation of an interaction site model for the cannabinoid analgesics.
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    International Journal of Quantum Chemistry 38 (1990), S. 189-206 
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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: Reaction fields are introduced to treat charge transfer excitations in chromophore systems. The methods are tested on the 9,9′-dianthryl dimeric molecule. The energy shift of the low energy component in dual fluorescence is a function of the polarity of the solvent and is correctly reproduced if an interpolation formula is used between the Mataga-Nishimoto and Ohno-Klopman approximations for the bi-centric γ integrals. The model is then applied to the special pair of the photosynthetic reaction center of Rhodopseudomonas viridis. This special pair is found to have an S1 state with dipolar character, corresponding to partial M ← L charge transfer. The bias is due to the bent shape of the M chromophore.
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    International Journal of Quantum Chemistry 38 (1990), S. XV 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 180
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    International Journal of Quantum Chemistry 38 (1990), S. 41-50 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Geometrical and electronic structures of chlorinated polyacetylene, (CHCl)x, a precursor of one-dimensional conjugated polyyne, have been investigated through experimental and theoretical approaches. Uniaxially oriented (CHCl)x films prepared by means of prolonged FeCl3-doping of mechanically stretched polyacetylene films have been subjected to analyses. Polarized infrared absorption spectra showed a dichroism to confirm an orientation of the (CHCl)x films. On the basis of x-ray diffraction analysis, a fiber period of the (CHCl)x has been determined to be 5.2 ± 0.1 Å. Ab initio crystal orbital calculations have been performed on structural models of infinite polymer systems for the (CHCl)x. Energy gradient optimization afforded a relative stability of three different configurational structures of poly-1,2-dichloroethylene which is assumed to be a planar zigzag all-trans conformation with respect to carbon-carbon single bonds, i.e., di-syndiotactic (dst) 〉 erythro-di-isotactic (e-dit) 〉 threo-di-isotactic (t-dit) structures. In addition, the fiber period of the ist structure was calculated to be 5.2 Å, coinciding with the experimental value of the (CHCl)x. In consequence, it has been demonstrated that the prolonged FeCl3-doping of polyacetylene causes a stereospecific chlorination to yield a stereoregular dst structure of the (CHCl)x. The present results imply that a usage of this sort of chlorination through the chemical doping should provide a novel reaction design, especially to control a stereospecificity of chlorination reaction.
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    International Journal of Quantum Chemistry 38 (1990), S. 375-382 
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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 partition of a molecule in different regions where virial relations are fulfilled has been proved in the literature to be a fruitful scheme for the description of molecular properties. However, the conceptual importance of this approach is somewhat diminished by the fact that nuclei are considered to occupy successive fixed positions. We show that this restriction can in fact be relaxed when such partitions of molecules are considered. The conditions that density functions must fulfill on the surfaces of such regions, when the nuclear motion is taken into account, are found. Using these conditions, we have examined general cases where the nuclear motion may introduce important modifications of the surfaces defined by the usual condition of zero flux on the one-particle density corresponding to the fixed nuclei model. Finally a further generalization is introduced by considering partitions of subsystems of particles of the system.
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    International Journal of Quantum Chemistry 38 (1990), S. 411-418 
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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 theoretical prediction of the absorption spectra in n-doped semiconductors due to donor pairs is extended further to investigate its contribution to the refractive index of phosphorus-doped silicon. Such a contribution yields a polarization catastrophe at the metal-nonmetal transition. The alternant molecular orbital with a many-valley character of the host material is used in the calculation. The results show good agreement with experiments.
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    International Journal of Quantum Chemistry 38 (1990), S. 475-479 
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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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    International Journal of Quantum Chemistry 38 (1990), S. 521-529 
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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 role of LixHy(CsxHy) molecules, which can be formed from seeding an alkali into a hydrogen plasma, is examined through ab initio calculations of the structure and stability of such species. The simplest alkali hydride, LiH, supports a bound anion for all internuclear separations. Larger structures, such as Li2H2 and Li3H, do not support stable anions but exhibit thermodynamic stability as gas phase molecules. Their possible roles in dissociative attachment to form H- and/or Li- is examined.
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    International Journal of Quantum Chemistry 38 (1990), S. 557-561 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In a recent work, March has considered the quantum-mechanical system of an arbitrary number of closed electronic shells around an atomic nucleus of charge Zae. March has assumed that the shells are filled by noninteracting electrons, moving in the bare Coulomb potential energy of V(r) = -Zae2/r. In this framework, the basic quantity is the total electron (number) density ρ(r), built by summing the (number) density ρn(r) of the nth closed electronic shell over the principal quantum number n. March has shown that Kato's theorem, (∂(r)/∂r)r = 0 = -(2Za/a0)ρ (r = 0), with a0 = h2/me2, is amenable to a spatially dependent generalization that can be expressed by ∂(r)/∂r = -(2Za/a0)ρs(r), where ρs(r) is the s-state contribution from the n closed electronic shells to ρ(r). The present work investigates the spatially dependent generalization of Kato's theorem for the Ne atom by making use of a variational density-functional approach and adopting several expressions for the kinetic-energy functional of the electrons. The intriguing question of identifying the “best” kinetic-energy functional is raised and discussed.
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    International Journal of Quantum Chemistry 38 (1990), S. 593-606 
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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 alternative configuration space scheme for ab initio polymer band structure calculations based on a Filon-type quadrature is proposed. This scheme avoids the explicit calculation and the storage of the “troublesome” Fourier transforms of the LCAO density matrix elements. Furthermore, the lattice sums (Coulomb and exchange) can be carried out up to infinity which removes the difficult problem of guessing limits to these summations. The potential of the proposed technique is illustrated by minimal basis set calculations on the infinite chains of H and He atoms.
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    International Journal of Quantum Chemistry 38 (1990), S. 187-195 
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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 report on the results of first-principles, SCF calculations of the electronic structure of the unrelaxed, substitutional, divalent Vanadium impurity in ZnSe. The effects of the surrounding ZnSe crystalline environment and of electron correlation on the SCF results reported here have been studied and are discussed.
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    International Journal of Quantum Chemistry 38 (1990), S. 225-240 
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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: Ab initio self-consistent field calculations are reported for electronic states of beryllium clusters comprised of 81 and 87 atoms. The clusters correspond to the tenth and eleventh coordination shells of a central Be with internuclear separations derived from the lattice constants of the bulk metal. Ab initio effective core potentials have been employed to replace the 1s electrons, therey reducing the complexity of the calculations. In addition, the use of the full D3h point group symmetry of the clusters results in a substantial reduction of the number of two-electron integrals that must be computed and processed. Binding energies, orbital energies, electric field gradient, nuclear-electrostatic potential, diamagnetic shielding constant, second moments, and Mulliken populations are calculated for selected electronic states. Systematic trends toward bulk behavior with increasing cluster size, as found in earlier cluster studies, continue to appear for the electric field gradient and quadrupole moment of Be81. Anomalous behavior, however, is observed in Be87. This is attributed to distortional effects due to the addition of atoms in planes above and below the Be81 cluster along the principal axis of symmetry.
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    International Journal of Quantum Chemistry 38 (1990), S. 665-678 
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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 INDO/1 semi-empirical method is used to analyze the hydroxylation of alkanes by models for the putative active species in cytochromes P-450. In the insertion of oxygen into the C—H bond by a M - O complex there are four atoms of particular interest, the metal and oxygen atoms of the oxidant plus the C and H atom of the C—H bond to be activated. With four atoms being studied (3N-6=), 6 degrees of freedom need to be described and a nearly intractable computational problem results. Thus, two reaction coordinates (CHI and OAT) were developed in an attempt to describe alkane hydroxylation in a chemically meaningful way. CHI (C—H insertion) is a measure of the extent to which the C—H bond has been activated and transferred to the O atom. OAT (oxygen atom transfer) is a measure of the extent to which the C—O bond in the alcohol product has formed. The models chosen for the present study are manganyl (MnO) and ferryl (FeO) porphyrin complexes. Experimental evidence for manganese prophyrin complexes shows that as the axial ligand is made a stronger donor the insertion of the O atom into the C—H bond becomes less concerted. The INDO/1 method was used to calculate the points on the PES determined by the changes in OAT and CHI. The results point to the axial ligand modifying the pathway by changing the relative electro-/nucleophilicity of the oxygen atom.
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    International Journal of Quantum Chemistry 38 (1990), S. 707-716 
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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: Fundamental aspects of the ensemble-density functional approach to the calculation of excited state energies are reviewed. Attention is given to the quasilocal density approximation for the equiensemble exchange-correlation energy functional. In particular, compared with experimental data, numerical results for the excitation spectrum of the helium atom produce an estimate of the deviations introduced by that approximation.
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    International Journal of Quantum Chemistry 38 (1990), S. 365-373 
    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 structures of the Li+ complexes with the bases NH3, H2O, HF, PH3, H2S, HCl, and HCN and with the corresponding deprotonated anions have been optimized using second-order Møller-Plesset perturbation theory with the 6-31+G(d,p) basis set. Single-point calculations on all species were performed using the 6-31+G(2d,2p) basis set using the following correlation methods: many-body (Møller-Plesset) perturbation theory at second (MP2), third (MP3), and fourth (MP4) order; the linearized coupled-cluster method (LCCM); the averaged coupled pair functional (ACPF); configuration interaction with all single and double excitations (CISD); and CISD with the Davidson and Pople corrections. The computed lithium ion affinities of the neutral bases vary between 15 and 40 kcal/mol, while the values for the anionic bases are between 145 and 190 kcal/mol. The effect of correlation on the lithium ion affinities varies between about -5 and +2 kcal/mol. For each neutral base, the lithium ion affinities computed with the various correlated methods agree with each other to between 0.1 and 1 kcal/mol. For the anions, the various correlated methods agree with each other to between 0.5 and 3 kcal/mol. Agreement with the available experimental data is within the estimated uncertainties in the latter. Structural differences between corresponding first- and second-row complexes reflect differences in the strength of the electrostatic interaction due to differences in the charge distribution.
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  • 192
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    International Journal of Quantum Chemistry 38 (1990), S. 405-410 
    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: Formula and analytic methods have previously been explored for the evaluation of multicenter molecular integrals over Slater-type orbitals by employing the Löwdin α-function approach. These procedures are greatly simplified by numerical integrations. The programming of this numerical approach is straight-forward and hence can serve as a check on future developments.
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  • 193
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    International Journal of Quantum Chemistry 38 (1990), S. 447-453 
    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 two-dimensional (2D) electron gas present in Si δ-doped layers in p-type GaAs is investigated through photoreflectance (PR) measurements performed at 300 and 77 K. The obtained spectra show oscillatory structures above the GaAs band gap which are ascribed to Franz-Keldysh oscillations. The decrease in the energy differences of the oscillations extrema with temperature indicates a reduction of the built-in electric field in the δ-doped region. The values of the electric field extracted from the experimental data are compared with those obtained from theoretical calculations based on a simultaneous self-consistent solution of the Schrödinger and Poisson equations. The agreement between theory and experiment strongly indicate that the observed oscillatory structures in the PR spectra are due to transitions to electron states of the 2D electron gas.
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  • 194
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    International Journal of Quantum Chemistry 38 (1990), S. 455-466 
    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: Binding energies for complexes of N2O with the acids H+, Li+, and HF have been computed using the following correlation methods: many-body (Møller-Plesset) perturbation theory at second (MP2), third (MP3), and fourth (MP4) order; the “quadratic CI” method with single and double excitations (QCISD) and with noniterative inclusion of triple excitations (QCISD(T)); the linearized coupled-cluster method (LCCM); the averaged coupled-pair functional (ACPF); configuration interaction with all single and double excitations (CISD); and CISD with the Davidson and Pople corrections. The convergence of the Møller-Plesset expansion is erratic, predicting that the terminal nitrogen is the preferred binding site for the complexes at the MP2 and MP4 levels, in disagreement with Hartree-Fock and MP3 and all other models (including the infinite-order QCI). The effect of triple excitations at MP4 and QCI is to destabilize complexes bound at O and stabilize those bound at N, but this effect is greatly overestimated at MP4 relative to QCI. Except for the LCCM result for N-protonated N2O, ACPF and LCCM bindin energies are similar to the QCISD values. The size-consistency error in the ACPF binding energies of the complexes of N2O with HF is about 0.5 kcal/mol. The CISD size-consistency error for these complexes is 23 kcal/mol, leading to negative binding energies when computed relative to isolated N2O and HF. The Davidson correction reduces the size-consistency error but still leaves the binding energies negative. The Pople correction produces positive but too small binding energies. However, all methods give consistenly good, essentially indistinguishable binding energies when computed relative to a similar calculation on a supermolecule of infinitely separated N2O and HF units. Multireference ACPF and CISD binding energies have also been obtained using MCSCF reference functions which correlate the highest occupied and lowest virtual pairs of π orbitals. The multireference binding energies are not greatly different from the single-reference values, and do not reverse the negative binding energies of the complexes with HF at the CISD level when computed relative to isolated molecules. When calculated relative to the infinitely separated supermolecule, the multireference binding energies of these complexes are virtually identical to the corresponding single-reference values.
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  • 195
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    International Journal of Quantum Chemistry 39 (1991), S. 71-91 
    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: The usefulness of the MS-Xα method for calculating molecular transition properties is investigated in a study of NO2. Two-dimensional potential surfaces are obtained in C2v symmetry for the X̃2A1/Ã2B2 system. Potential curves that approximately follow the asymmetric stretch coordinate are also presented: On investigation, no symmetry-broken solutions were found. At all nuclear geometries investigated, the muffin-tin sphere radii were chosen by an energy-minimization technique. The inclusion of corrections to the muffin-tin form of the Xα total energy is shown to be essential, if the computed surfaces are to be qualitatively correct. The transition dipole moment connecting the two states is evaluated as a function of nuclear coordinates, using the transition-state approximation. Finally, the radiative decay of a single vibronic level in NO2 is modeled and the relevance to experiment is discussed.
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  • 196
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    International Journal of Quantum Chemistry 39 (1991), S. 131-147 
    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: Nonempirical quantum chemical method Hartree-Fock-Roothan LCAO SCF MO in a two-exponent Dunning basis with the use of an extended set of Gaussian functions by Huzinaga-Dunning with consideration of electron correlation according to the Meller-Plesset theory of excitations of the second order was used to study monohydrates of Li+, Na+, K+, and HCOO- ions. The indicated basis was supplemented with polarization functions of d-type on the O atom and of p-type on the hydrogen atom as well as with diffusion functions of p-type on the oxygen atom. It has been found that binding energies of the water molecule with Li+, Na+ appeared to be higher and with K+ lower than with HCOO- · H2O. Potential curve shapes of K+ + H2O and HCOO- + H2O reactions are shown to be similar. The molecular mechanism of K+ channel selectivity of an excitable membrane is explained on the basis of the obtained calculations.
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  • 197
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    International Journal of Quantum Chemistry 38 (1990), S. 645-653 
    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 ab initio self-consistent-field molecular orbital (SCF-MO) procedure has been used to compute the average local ionization energies [Ī(r)] of some strained molecules on three-dimensional surfaces defined by the contour of electronic density equal to 0.002 electrons/bohr3. Ī(r) is rigorously defined within the framework of SCF-MO theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecule. Thus, the positions of the smallest Ī(r) values (ĪSmin) on the surfaces of molecules are the sites expected to be the most reactive to electrophiles. We find ĪSmin, near the C—C bond midpoints of saturated three-membered, but not foru-membered, hydrocarbon rings. These ĪS,min are interpreted as reflecting the “σ-aromatic” character of the former. Our Ī(r) data effectively provide a “fingerprint” characterizing saturated three-membered rings, which is useful, for example, in analyzing molecules such as [1.1.1] propellane. Our results for the latter are consistent with the interpretation of it having a biradical character, as has been suggested earlier.
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  • 198
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    International Journal of Quantum Chemistry 38 (1990), S. 683-692 
    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 interaction (CI) method is presented in which the space of configuration state functions expands in every iteration until all numerically significant configurations are taken into account. The algorithm is based on a new scheme of addressing unstructured sets of configurations and on the projection into the space of intermediate spin-adapted configurations with 2 electrons fixed on “ghost” orbitals.
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  • 199
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    International Journal of Quantum Chemistry 38 (1990), S. 693-699 
    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 fundamental mechanism responsible for high temperature superconductivity in the copper oxides is still not known. In view of this, it is important to consider how best to account for the normal state properties of these materials in the hope of characterizing the excitations which are relevant to the superconducting instability. Results of local density functional approximation (LDA) calculations are presented of the electronic density response, which yields predictions of phonon frequencies and electron-phonon interaction strength as well as structural parameters and structural instabilities. Structural properties and phonon frequencies are in excellent agreement with experiment in both La2-xMxCuO4 and YBa2Cu3O7. Theoretical evidence is also presented for strong nonlocal contributions to the electron-phonon coupling in these materials.
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  • 200
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    International Journal of Quantum Chemistry 38 (1990), S. 721-732 
    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: Geometries and harmonic force constants were calculated for trans- and gauche-butadiene and for trans- hexatriene using SCF and electron correlation (ACPF) methods. By comparing the results of these two procedures trends in the errors of the SCF values are discussed. Although diagonal SCF force constants are usually too large in relation to correlated results, the opposite is true (in absolute value) for several off-diagonal force constants referring to the carbon backbone. Scaling techniques are used in order to correct for these deficiencies and also for anharmonicity effects. On that basis scaled SCF force fields and vibrational frequencies were calculated for trans-polyenes up to 14 carbon atoms and for the infinite chain.
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