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  • 1995-1999
  • 1990-1994  (403)
  • 1990  (403)
  • Computational Chemistry and Molecular Modeling  (403)
  • Nuclear reactions
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  • 1995-1999
  • 1990-1994  (403)
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  • 101
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 2 Ill.
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  • 102
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 5 Tab.
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  • 103
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    New York, NY : Wiley-Blackwell
    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
    Type of Medium: Electronic Resource
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  • 104
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    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
    Type of Medium: Electronic Resource
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  • 105
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    New York, NY : Wiley-Blackwell
    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
    Additional Material: 5 Ill.
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  • 106
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 107
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    New York, NY : Wiley-Blackwell
    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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  • 108
    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.
    Additional Material: 8 Ill.
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  • 109
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    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 2 Ill.
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  • 110
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 37 (1990), S. 121-123 
    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: 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.
    Additional Material: 2 Ill.
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  • 111
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 37 (1990) 
    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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  • 112
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    International Journal of Quantum Chemistry 37 (1990), S. 51-68 
    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: 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.
    Additional Material: 2 Tab.
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  • 113
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    International Journal of Quantum Chemistry 37 (1990), S. 423-436 
    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: 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.
    Additional Material: 9 Ill.
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  • 114
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    International Journal of Quantum Chemistry 37 (1990), S. 113-120 
    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: 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.
    Additional Material: 4 Ill.
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  • 115
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    International Journal of Quantum Chemistry 37 (1990), S. 85-94 
    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: 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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  • 116
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 37 (1990) 
    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 37 (1990), S. 797-809 
    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 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).
    Additional Material: 2 Ill.
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  • 118
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    International Journal of Quantum Chemistry 37 (1990), S. 821-821 
    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. 465-483 
    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: 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.
    Additional Material: 3 Ill.
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  • 120
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    International Journal of Quantum Chemistry 37 (1990), S. 517-528 
    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 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.
    Additional Material: 4 Tab.
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  • 121
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    International Journal of Quantum Chemistry 37 (1990), S. 529-537 
    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 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.
    Additional Material: 5 Ill.
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  • 122
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    International Journal of Quantum Chemistry 37 (1990), S. 539-546 
    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: 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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  • 123
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    International Journal of Quantum Chemistry 37 (1990), S. 609-609 
    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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  • 124
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    International Journal of Quantum Chemistry 37 (1990), S. 631-653 
    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: 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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  • 125
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    International Journal of Quantum Chemistry 37 (1990), S. 679-683 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the present 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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  • 126
    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 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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  • 127
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    International Journal of Quantum Chemistry 38 (1990) 
    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 38 (1990), S. 11-23 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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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  • 129
    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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  • 130
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    International Journal of Quantum Chemistry 38 (1990), S. 97-97 
    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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  • 131
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    International Journal of Quantum Chemistry 38 (1990), S. 121-128 
    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: (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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  • 132
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    International Journal of Quantum Chemistry 38 (1990), S. 447-460 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the 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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  • 133
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    International Journal of Quantum Chemistry 38 (1990), S. 119-120 
    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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  • 134
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    International Journal of Quantum Chemistry 38 (1990), S. 139-147 
    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: 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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  • 135
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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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  • 136
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    International Journal of Quantum Chemistry 38 (1990), S. 221-224 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The quantum 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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  • 137
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    International Journal of Quantum Chemistry 38 (1990), S. 245-252 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    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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  • 138
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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 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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  • 139
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    International Journal of Quantum Chemistry 38 (1990), S. 329-338 
    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 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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  • 140
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    International Journal of Quantum Chemistry 38 (1990), S. 357-372 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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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  • 141
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    International Journal of Quantum Chemistry 38 (1990), S. 699-711 
    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: 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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  • 142
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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
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 143
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    International Journal of Quantum Chemistry 38 (1990), S. 819-830 
    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 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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  • 144
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    International Journal of Quantum Chemistry 38 (1990), S. 161-171 
    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: 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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  • 145
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    International Journal of Quantum Chemistry 38 (1990), S. 015-028 
    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: 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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  • 146
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    International Journal of Quantum Chemistry 38 (1990), S. 675-689 
    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 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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  • 147
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    International Journal of Quantum Chemistry 38 (1990), S. 119-131 
    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 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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  • 148
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    International Journal of Quantum Chemistry 38 (1990), S. 189-206 
    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: 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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  • 149
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    International Journal of Quantum Chemistry 38 (1990), S. XV 
    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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  • 150
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    International Journal of Quantum Chemistry 38 (1990), S. 41-50 
    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: 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 
    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 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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  • 152
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    International Journal of Quantum Chemistry 38 (1990), S. 411-418 
    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 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 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 2 Ill.
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  • 154
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    International Journal of Quantum Chemistry 38 (1990), S. 521-529 
    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 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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  • 155
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    International Journal of Quantum Chemistry 38 (1990), S. 557-561 
    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 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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  • 156
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    International Journal of Quantum Chemistry 38 (1990), S. 593-606 
    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 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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  • 157
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    International Journal of Quantum Chemistry 38 (1990), S. 187-195 
    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 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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  • 158
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    International Journal of Quantum Chemistry 38 (1990), S. 225-240 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio 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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  • 159
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    International Journal of Quantum Chemistry 38 (1990), S. 665-678 
    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 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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  • 160
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    International Journal of Quantum Chemistry 38 (1990), S. 707-716 
    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: 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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  • 161
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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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  • 162
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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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  • 163
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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.
    Additional Material: 3 Ill.
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  • 164
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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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  • 165
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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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  • 166
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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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  • 167
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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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  • 168
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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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  • 169
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    International Journal of Quantum Chemistry 37 (1990), S. 449-463 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two different color-related applications of the Pariser-Parr-Pople method are described. The first one deals with an explanation of the UV-visible absorption spectrum of the curcumin pigment in turmeric and the color change of curcumin upon addition of OH-. The other application deals with blue and red shifts in poly-diacetylene to polybutatriene transformation. The applications of the FEMO and PPP methods to curcumin facilitate assignment of the two observed bands of curcumin (UV and visible) to the π → π* (UV) and n → π* (visible) transitions, respectively. A graph-theoretical formulation of the PPP method is also considered.
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  • 170
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    International Journal of Quantum Chemistry 37 (1990), S. 509-516 
    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 purpose of this article is the comparison of generalized valence bond (GVB) and the alternant molecular orbital (AMO) techniques both in the localized and delocalized form. The model we have chosen is the PPP model of cis-butadiene which is relatively simple and transparent. For equal bond lengths the AMO using delocalized orbitals is better than the GVB. For physical distances of the atoms these methods are of comparable accuracy. With the extension of the length of the central bond the GVB becomes better and is exact in the limit of two ethylenes. The AMO method using localized orbitals always gives the worst result of the three. In the limit of two ethylenes AMO in the localized and delocalized forms give the same error.
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  • 171
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    International Journal of Quantum Chemistry 37 (1990), S. 547-558 
    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: Semiempirical molecular orbital calculations within a modified PPP method, the Peel method, have been performed in order to study the electronic structure and properties of porphyrins and phthalocyanines. It is found that the calculations can reproduce the results from more sophisticated ab initio calculations. It is found that the electronic reactions may take place at the metal atom and in the axial direction. The role of the porphine plane seems to be that of an electron buffer, keeping the electronic population in the porphyrin plane constant. A mechanism is suggested where electron transfer reactions can take place by change in the molecular geometry, as for example by vibrations.
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  • 172
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    International Journal of Quantum Chemistry 37 (1990) 
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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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  • 173
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    International Journal of Quantum Chemistry 37 (1990), S. 611-629 
    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 carried out a computational study of hydrazine and five of its 1,1-dimethyl derivatives, focusing on their electrostatic potentials and relative bond strengths. Our approach has involved the calculation of ab initio self-consistent-field molecular orbital wave functions and molecular properties using the GAUSSIAN 82 system of programs. The electrostatic potentials of the hydrazines possess negative regions of varying sizes and strengths associated with the nitrogens of the α-diamino linkages. Through an analysis of the positions of the most negative potentials of these regions, we have obtained directly the dihedral angles between the nitrogen lone pairs in these systems. Our use of the electrostatic potential to obtain these angles is a direct and general approach, in contrast to indirect procedures used in the past. We find this dihedral angle to be close to 90° in hydrazine, with variations in the substituted hydrazines that depend on the nature of the substituents. A highly polar structure is found for 1-chloromethyl-1-methylhydrazine, which involves a delocalization of electronic charge from the substituted nitrogen towards the CH2Cl group. We find that substituents able to withdraw significant amounts of electronic density from the central nitrogen lone pair regions, either through resonance or by induction, have a slight bond strengthening effect on the central N-N bond. This is attributed to a decrease in the repulsion between the weakened nitrogen lone pair regions. The difficulties encountered in seeking the controlled oxidation of hydrazine to nitro derivatives may be due, in part, to the fact that two factors which would favor this, highly negative nitrogen potentials and strong N-N bonds, are opposing in nature.
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  • 174
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    International Journal of Quantum Chemistry 37 (1990), S. 685-699 
    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: Earlier definitions of valencies of atoms, molecules, and molecular orbitals are extended to configuration interaction (CI) wave functions. Using these definitions, valencies both at equilibrium and nonequilibrium geometries of molecules are calculated at the CI level and compared with non-CI results. CI valency correlation diagrams are obtained. Valency variation with bond length using correlated wave functions is found to behave properly unlike in the case of SCF wave functions.
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  • 175
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    International Journal of Quantum Chemistry 37 (1990), S. 729-746 
    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 adiabatic potential energy surfaces (PES) which are most likely to be involved in the elementary mechanism presiding over charge-exchange and direct inelastic collisions between O2 molecules and collimated beams of protons are discussed. The general behavior of Diatomics-in-molecule (DIM) model interactions is analyzed in great detail as a function of the molecular vibrational coordinate and of the other internal nuclear coordinates. The general features of the lower two PES are discussed, and the corresponding nonadiabatic coupling terms between these surfaces are also computed and analyzed. These model results turn out to provide very useful indications on the specific dynamical features that are to be considered responsible for the inelastic, vibronic transitions observed in the target molecule during collisional experiments.
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  • 176
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    International Journal of Quantum Chemistry 37 (1990), S. 785-796 
    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: Extensive SCF-LCAO-MO variational and perturbative configuration interaction (CI) calculations framed within an effective core potential approximation have been performed to determine the two experimentally observed geometrical isomers of Ag O2 and the interconversion route between them. These structural forms, associated to the ground-state local minima, yield virtually the same energy, and their spontaneous interconversion is strongly indicated, which agrees fairly well with the experimental measurements. The reaction Ag + O2 → Ag O2 was theoretically analyzed along a CI fully optimized energy pathway for the ground and various excited states, within C2v and Cs symmetry. Although a tight-ion pair (Ag+ O2-) character is predicted for the ground state at the equilibrium geometries, its dissociation leads to neutral rather than to ionic fragments. The study of the reaction path within Cs symmetry shows an avoided crossing between the ground state and another 2A″ potential curve where the former correlates adiabatically with the reactants Ag(2S) + O2(1Δg). This indicates that the formation of the complex proceeds via a reactive state of molecular oxygen. The higher 2A″ electronic curves correlate with the metal 2P excited state, and the oxygen binding is found to be less favorable. The present results are shown to have an important bearing on the experimentally known catalytic properties of oxygen adsorbed on silver surfaces.
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  • 177
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    International Journal of Quantum Chemistry 38 (1990), S. 1-10 
    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: Based on completely optimized molecular geometries of the photophysically relevant electronic states, the corresponding vibrational energies and wave functions and the theoretical rate constants for the internal conversion processes of azulene within the Sp, term system are presented. The influence of the promoting and accepting modes on the radiationless deactivation rates is discussed. An analysis of the active nonradiative modes is given. In context with the respective theoretical radiative rates, the efficiency of the different azulene fluorescences is interpreted.
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  • 178
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    International Journal of Quantum Chemistry 38 (1990), S. 55-74 
    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: Sequences of total energies of trans-polyacetylene were obtained by either finite cluster or crystal-orbital calculations. Several different extrapolation methods were employed to improve the convergence of these two sequences. The properties of the extrapolation algorithms are discussed with emphasis on efficiency and reliability for short strings of physical input data. Four decimal digits were gained by extrapolation in the case of the finite cluster energies, but, at most, 2 decimal digits in the case of the crystal-orbital energies. The extrapolation of the finite cluster energies gave results that are almost as good as the results produced by the extrapolation of the crystal-orbital energies, and that are better than the unextrapolated crystal-orbital energies.
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  • 179
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    International Journal of Quantum Chemistry 38 (1990), S. 95-95 
    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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  • 180
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    International Journal of Quantum Chemistry 38 (1990), S. 181-189 
    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 direct reaction field (DRF) method, developed to incorporate the effects of a (large) semiclassical environment into the Hamiltonian of a quantum mechanical system, is briefly reviewed. It is shown that the DRF method behaves - at least - like a supermolecule SCF calculation. With the water dimer as an example, the similarity with the SCF procedure is demonstrated, and an application to the interaction between the active site of papain and the remaining 3000 or so atoms of this protein shows the inadequacy of dielectric constant models and the necessity of including atomic polarizabilities in model force fields.
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  • 181
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    International Journal of Quantum Chemistry 38 (1990), S. 225-229 
    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
    Notes: The dependence of the long-wave absorption-band intensity on the structural peculiarities of a number of polymethine dyes has been investigated using the PPP method. It has been shown that the analyzed dependence is caused by the electronic properties of the substitutes.
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  • 182
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    International Journal of Quantum Chemistry 38 (1990), S. 253-278 
    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 graph-theory model of synthons is suggested. A synthon is a special kind of the molecular graph in which some vertices are distinguished from other ones, and they are called the virtual vertices. The most important property of the synthons is that the constraint of strict stoichiometry is removed and the virtual vertices formally correspond to functional groups that are not closely specified.
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  • 183
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    International Journal of Quantum Chemistry 37 (1990), S. 35-50 
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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: Different methods of averaging of energy over the states of electronic configurations γN (nγ = 1, 2, 3 and N = 1, 2, …, 2nγ - 1) leading to Roothaan' energy expression are considered. The consequent values of vector coupling coefficients (VCC) in energy functionals for various states as well as for average values of energy are presented.It is shown also that in molecular systems of cubic and tetragonal symmetry having electronic configurations tN (N = 2-4) and e2 there exist states for which VCC are dependent on the choice of basis set of degenerate open-shell molecular orbitals. The origin of such “non-Roothaan” terms and peculiarities of its calculation by the restricted Hartree-Fock method are discussed.
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    International Journal of Quantum Chemistry 37 (1990), S. 111-111 
    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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  • 185
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    International Journal of Quantum Chemistry 37 (1990), S. 315-315 
    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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  • 186
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    International Journal of Quantum Chemistry 37 (1990), S. 373-387 
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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 Born-Oppenheimer spin-conserving Hamiltonian operator expressed in terms of the Lie algebra generators is partitioned according to the number of connected lines in the corresponding graphical representation. A simple derivation of some basic relations with the generalized Brillouin theorem, HF approach, and random phase approximation is presented.
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  • 187
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    International Journal of Quantum Chemistry 37 (1990), S. 95-102 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The many-body expansion of the potential energy function of elemental clusters is examined in general terms in regard to its convergence for microclusters and the bulk phase. The systems Ben and Lin are examined in detail. For Li it is clear that the many-body expansion has no low-order convergence, but it is shown that a potential of the form \documentclass{article}\pagestyle{empty}\begin{document}$$ V = aV^{(2)} + bV^{(3)} $$\end{document} gives good binding energies for Lin (3〈n≤9) and also a good value for the heat of atomization of the bcc crystal.
    Additional Material: 1 Ill.
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  • 188
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    International Journal of Quantum Chemistry 37 (1990), S. 485-495 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is nearly 40 years since the PPP method was proposed, and we demonstrate here that the method is still active in studying the electronic structures of very large conjugated systems and/or periodic systems. The kink (solition) problems in polyacetylene are solved with the UHF-PPP Hamiltonian, demonstrating that there appear several new features which have not been obtained without considering the interelectronic interactions of the PPP type. Next, the electron correlation effect in polyacetylene is discussed through the crystal orbital-PPP CI calculations. Finally, three-dimensional crystal orbital-CNDO calculations on graphite and black phosphorus are reported.
    Additional Material: 10 Ill.
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  • 189
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    International Journal of Quantum Chemistry 37 (1990), S. 589-597 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Previous work has shown that the heats of formation of unstrained conjugated molecules can be reproduced with surpring accuracy by a semiempirical SCF MO treatment (DEWAR-PI) based on the Pariser-Parr-Pople (PPP) π SCF MO approximation. The original version failed to allow for ring strain. This deficiency has now been remedied in a new version (DEWAR-PI2).
    Additional Material: 1 Ill.
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  • 190
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    International Journal of Quantum Chemistry 37 (1990), S. 559-588 
    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 Pariser-Parr-Pople (PPP) method has revealed rich effects of the π electron Coulomb interaction in (CH)x. We discuss results obtained by the PPP model with the unrestricted Hartree-Fock (UHF) approximation and we show that concepts introduced by the PPP-UHF model give a new insight on Franck-Condon-type electronically excited states in (CH)x. In the first part of this paper, a brief review of the PPP-UHF model is given. We introduce phase and amplitude variables and show that solitons in the PPP-UHF model are phase excitations. In the second part of this paper, the effective Hamiltonian describing phase excitations is derived. We show that the phase Hamiltonian derived by the time-dependent Hartree-Fock approximation is the classical limit of the Hamiltonian derived by the bosonization method of Tomonaga, Luther, and others. The phase Hamiltonian has a family of new soliton solutions. These solitons are pure electronic excitations and correspond to the exciton and the magnon in (CH)x.
    Additional Material: 9 Ill.
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  • 191
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    International Journal of Quantum Chemistry 37 (1990), S. 241-248 
    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: Many-body perturbation theory up to second-order in the Møller-Plesset partitioning has been used to calculate the quasiparticle band structure of hexagonal boron nitride, treated as a periodic two-dimensional system. It was found that correlation leads to an essential narrowing of the fundamental gap. The exciton spectrum has been calculated using the first order irreducible vertex part with Hartree-Fock and quasiparticle band structure data. It was found that the exciton binding energy amounts to about 10% of the fundamental gap (for the first singlet excitation).
    Additional Material: 1 Ill.
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  • 192
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    International Journal of Quantum Chemistry 37 (1990), S. 291-307 
    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: Separability of the Hamiltonian describing a two-center, one-electron Coulombic system in the prolate spherical coordinates allows for a detailed comparison between the partitioning of electronic properties based on Bader's quantum theory of atoms in molecules and the newly developed GAPT partitioning. The one-electron systems can serve as the simplest models of the covalent and ionic bonds. The GAPT partitioning correctly describes the effects associated with the infinite range of Coulombic interactions.
    Additional Material: 19 Ill.
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  • 193
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    International Journal of Quantum Chemistry 37 (1990) 
    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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  • 194
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    International Journal of Quantum Chemistry 38 (1990), S. 311-319 
    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 growth-regulating factor MDGI belongs to the fatty acid binding proteins of which the amino acid sequence is known. Starting from hydrodynamic and dichroic measurements which yield estimations about mass, diameter, thickness, and content of α and β structure of the MDGI protein, we used theoretical methods to calculate the structures and electrostatic properties of selected β-chain sequences. The hydrophobic character of possible binding sites for fatty acids is demonstrated by the molecular electrostatic potential and field of this structures. A nonstandard turn element, formed by an H-bridge between polar side chains, Lys-Leu-Gly-Val-Glu, which connects two extended β chains, was found. Bound oleic acid could be able to stabilize such turn structure, which seems to appear also at the entrance of the cavity of two other proteins. The optimization of an oleic acid-turn van der Waals complex results in an interaction energy of about -50 kJ/mol originating mainly from the dispersion term and shows the participation of the four hydrophobic side chains Phe 64, Leu 66, Val 68, and Phe 70. Of these, at least Phe 64 and Phe 70 are known as possible binding sites of fatty acids.
    Additional Material: 1 Ill.
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  • 195
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    International Journal of Quantum Chemistry 38 (1990), S. 351-356 
    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: Theoretical 1s core-electron binding energies are presented for Al and Ar atoms in free space and in AlAl12, AlAl12Al6, and ArAl12 clusters. The binding energies have been calculated by the self-consistent field Xα scattered-wave (SCF Xα SW) method using various exchange parameters and different atomic-sphere overlaps. The atom/cluster binding-energy shifts have been obtained both from the Slater's transition-state energies and from the total-energy differences; these values are in better agreement with each other if calculated with proper overlapping than if with nonoverlapping spheres. A comparison with available experimental and theoretical data is given as well.
    Additional Material: 5 Tab.
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  • 196
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    International Journal of Quantum Chemistry 38 (1990), S. 373-381 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The quantum (ħ =1) and semiclassical (ℏ → 0) Wigner distribution functions were calculated for some anharmonic oscillators using a direct phase-space variational method.
    Additional Material: 6 Ill.
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  • 197
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    International Journal of Quantum Chemistry 38 (1990), S. 85-92 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio SCF were performed on the trans and gauche rotamers of succinonitrile (SN) and on the monohydrated rotamers to determine the most probable hydration site(s) of water with SN and the relative strengths of these interactions and to examine the effect of one water molecule on the relative stability of the rotamers. MOPAC was used to determine the location of water on the rotamers of SN; the geometries of the hydrated rotamers were then optimized at the HF/4-31G level followed by single-point calculations at the HF/6-31G and HF/6-31G* levels. The results show that trans SN is more stable than is gauche SN by 1.2 kcal/mol (at the HF/4-31G level with zero-point energy corrections) and 0.8 kcal/mol (at the HF/6-31G* level, excluding zero-point energy corrections). The hydrogen bond formed between the methylene hydrogen of trans SN and the oxygen of water is more stable than is the hydrogen bond formed between the nitrogen of SNt and the hydrogen of water by 0.63 kcal/mol. The most stable hydrogen-bonding interaction for the gauche rotamer occurs for the methylene - OH2 hydrogen-bonding interaction, which also has the largest hydration energy (ΔE = -4.39 kcal/mol), but it is less stable than is the most stable trans hydrogen-bonding interaction (SNt - OH2) by 0.63 kcal/mol. Our results suggest that the trans rotamer of SN should associate with more water molecules than should the gauche. The relative rotamer stability between the trans and gauche rotamers of SN decreases by 0.2 kcal/mol (at the HF/6-31G* level) when water is hydrogen bonded to the methylene group of SN. A mechanism is also proposed to explain the phase-separation behavior of this system.
    Additional Material: 3 Ill.
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  • 198
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    International Journal of Quantum Chemistry 38 (1990), S. 499-499 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 199
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    International Journal of Quantum Chemistry 38 (1990), S. 551-557 
    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: R. A. Davidson's rules for splitting an n-fold rotationally symmetric graph can be derived from a unitary transformation on the adjacency matrix. The McClelland and D'Amato rules are special cases with n = 2 and n = 3, respectively.
    Additional Material: 3 Ill.
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  • 200
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    International Journal of Quantum Chemistry 38 (1990), S. 585-596 
    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: Within the framework of the general extended geminal model, two new approximate models, EXGEM7 and EXRHF3, are introduced. Compared with previous models, these new models imply a more sophisticated approximation of the four-electron terms {∊KL}. The models are tested by calculations on the beryllium atom and the HF molecule.
    Additional Material: 4 Tab.
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