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  • Computational Chemistry and Molecular Modeling  (26)
  • Wiley-Blackwell  (26)
  • 1990-1994  (26)
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  • 1990  (26)
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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 153-166 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A pair-excitation-multiconfigurational-self-consistent-field (PEMCSCF) study with 10 configurations has been carried out on 3-methyl-1,2,4-oxadiazolin-5-one (3-methyl pseudosydnone) which has normal covalent bonds but is isoelectronic with 3-methyl sydnone for which no single covalent valence bond structure can be drawn. The PEMCSCF results were found to be very similar for the two isoelectronic compounds and an attempt was made to represent the bonding of the sydnone in only two simple molecular-orbital-valence-bond (MOVB) configurations using a Luken-type correlating orbital. A compact minimum basis of Slater-Transform-Preuss functions fitted by six cartesian gaussian orbitals (STP-6G) is used to express the compact representation for chemical comparison to experiment. The molecular geometry was gradient-optimized at the single determinantal level using a 6-31G extended basis set. “Orthonormal Resonance Analysis” is given for the two dominant configurations by noting that the Boys-Reeves configuration interaction algorithm is identical to the Pauling valence bond algorithm except for use of orthogonal molecular orbitals instead of atomic hybrid orbitals. This equivalence permits use of PEMCSCF atomic charges, bond orders, and dipole moments in a slightly modified valence bond interpretation of the unusual bonding of this compound. Although forcing a larger PEMCSCF problem into a minimum basis and only two configurations raises the energy, chemical interpretation is simplified. This comparison between a mesoionic compound and an isoelectric compound with normal bonding offers new understanding of the mesoionic effect.
    Additional Material: 3 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 173-179 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This short review concerns the protonation of Schiff bases by halogenic or carboxylic acids with a bias toward the chromophore of visual and bacterial rhodopsins. It is pointed out that the weak acids available in these pigments could not protonate the retinyl Schiff base 100% and that a supporting mechanism is needed to achieve full protonation. Our Fourier transform infrared studies both at room and low temperatures relating to this problem are summarized, and the propable role of water is emphasized.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 501-520 
    ISSN: 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, and limitations, of the fundamental physical quantities used in our current system of dosimetry for the protection of individuals against the hazardous effects of ionizing radiation is discussed briefly. A major limitation is the inability to correlate biological data, in a unified way, as a function of linear energy transfer with the consequent necessity for quality factors.From consideration of the various interaction processes undergone by charged particles in the equilibrium slowing down spectra generated by the incident radiation field, it is shown that good correlation of biological effects, for all radiation types, can be achieved in terms of linear primary ionization. The implication is that delta-ray effects play at most a very minor role, at moderate fluences, and that, consequently, the absorbed dose is an unsuitable parameter for describing radiation effects. Structure in the radiosensitive targets is observed to have a critical dimension of about 2 nm. It occurs only when double-stranded DNA is present and the magnitude of the inactivation probability is consistent with double-stranded breaks being the significant lesion. Calculation is made of the yield of lesions as a function of the mean free path for primary ionization.Differential spectra of radiation quality are obtained for monoenergetic electrons (0.2 keV to 30 MeV); for characteristic X-rays from carbon, aluminium, neon, potassium, copper, silver, and tungsten; for 50 kV X-rays, 250 kV X-rays, 241Am, 137Cs, and 60Co gamma rays; and for neutrons with energies between 0.1 and 50 MeV. Damage by electrons is shown to be predominant at the ends of their tracks between 50 and 200 eV, whereas this is not so for the proton recoil spectrum generated by neutrons above 1 MeV.Proposals are made for a unified system of dosimetry that is independent of radiation type and that obviates the need for quality factors. The method is thought to be of general applicability to irradiations by nuclides incorporated into mammalian cells, by ingested emitters, and by external sources.
    Additional Material: 6 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 717-719 
    ISSN: 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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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 141-151 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional calculations - with a local density approximation for the exchange and correlation energy - provide a numerically efficient method for determining total energy variations in molecules. Coupled with molecular dynamics techniques such as simulated annealing, they provide a scheme for probing large regions of configuration space in molecules. We outline the main features of the method and apply it to different ring isomers of S7O, S8, S7Se, Se8, and O8.
    Additional Material: 2 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    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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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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.
    Additional Material: 6 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    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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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 779-797 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new constructive approach for deriving a quantum field chemistry (QFC) is proposed. As a matter of fact, the approach is a direct application of the concept of spontaneously broken symmetry of a free-electron-field vacuum to the exact definition of a condensed-state chemical microstructure. The main idea is to identify the properly modified ground states of the vacuum with ground states of some compact quasi-molecular systems condensed in a set of “kink”-bounded molecular {vα} subspaces. Phase transitions of the electron vacuum are as usually expressed in terms of quantum order parameter set {φα1(x) = [ρα1(x)]1/2 exp(iλα)}, which defines a single electron densities ρα1(x)=nα(x)/Nα (here Nα=∫nα(x) dx) in exact molecule ground states ρα1(x)=|α1(x)|2. The order parameters are obtained by the self-consistent procedure of minimizing the ground-state energy Eα=Nα∊α-1(N) for each open molecular “compacton” with respect to the number of electrons Nα and an average single-electron energy ∊α-1(Nα). Account is taken of topological definitions of the various molecular constituents: atoms, atomic functional groups, molecules, and their clusters. The stability of diverse clusters is investigated by the method of approximating quasi-particle density functional (AQDF). It gives particular attention to the description of peculiar intermolecular clusters (Mn) composed with single-atom molecules (M1=A). Such clusters may be used to simulate some active centers that bear the responsibility for strong effects of nonlinearity and dissipation in condensed states. Some results of AQDF-calculation of small active Rh-clusters (n=2, 3, 4) are taken under consideration to illustrate that such clusters resemble neither fragments of bulk solids nor molecules in a gas.
    Additional Material: 2 Ill.
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