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  • 1980-1984  (1,668)
  • Computational Chemistry and Molecular Modeling  (1,667)
  • Nuclear reactions
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  • 1
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Most of the existing calculations of relativistic effects in many-electron atoms or molecules are based on the Dirac-Coulomb Hamiltonian HDC. However, because the electron-electron interaction mixes positive- and negative-energy states, the operator HDC has no normalizable eigenfunctions. This fact undermines the quantum-theoretic rationale for the Dirac-Hartree-Fock (DHF) equations and therefore that of the relativistic configuration-interaction (RCI) and multiconfiguration Dirac-Fock (MCDF) methods. An approach to this problem based on quantum electrodynamics is reviewed. It leads to a configuration-space Hamilton H+U which involves positive-energy projection operators dependent on an external potential U; identification of U with the nuclear potential Vext corresponds to use of the Furry bound-state interaction picture. It is shown that the RCI method can be reinterpreted as an approximation scheme for finding eigenvalues of a Hamiltonian H+U, with U identified as the DHF potential; the theoretical interpretation of the MCDF method needs further clarification. It is emphasized that if U differs from Vext one must consider the effects of virtual-pair creation by the difference potential δU = Vext - U; an approximate formula for the level-shift arising from δU is derived. Some ideas for dealing with the technical problems introduced by the projection operators are discussed and relativistic virial theorems are given. Finally, a possible scheme for adapting current MCDF methods to Hamiltonians involving projection operators is described.
    Additional Material: 3 Ill.
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  • 2
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A recently proposed orbital optimization technique based on the use of the inverse of the level-shifted Fock operator is successfully applied to a large number of pathologically divergent cases. The possibility of eliminating convergence problems by adopting a similar modification in the operation of the orthogonal gradient method is also successfully tested.
    Additional Material: 1 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 817-851 
    ISSN: 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 band structure of porphyrinatonickel(II) (2) has been studied by means of crystal orbital calculations that are based on the tight-binding approximation; the computational framework is a recently developed INDO model for transition metal compounds of the 3d series. The porphyrinato polymer has been studied in an eclipsed arrangement (2a) and in a staggered conformation (2b) where neighboring layers are rotated by 41°. The total energy of the metallomacrocycle has been decomposed into one- and two-center contributions; the latter interaction parameters have been fragmented into physically feasible resonance, exchange, and classical electrostatic (electron-electron, electron-core, core-core) interactions. It is shown that individual two-center potentials between atoms in neighboring layers are prevailingly determined by the electrostatic interaction energy. The NiNi coupling in the chain is highly repulsive; important stabilizing interactions are predicted between the 3d center of one cell and the electronegative N atoms in the neighboring layers. Stabilizing and destabilizing electrostatic interaction potentials largely compensate each other; the net stabilization in the polymer comes from the accumulation of resonance and exchange increments. The unoxidized Ni(II) porphyrinato polymer is an insulator. Several ligand bands (π, σ, and lone-pair) are predicted on top of bands with significant Ni 3d admixtures; the conduction band of the unoxidized strand is of ligand π* character. The dense manifold of ligand states in the vicinity of the Ni 3d states (3dz2, 3dx2-y2, 3dxz/3dyz) prevents the formation of bands in the polymer that are strongly localized at the 3d center. Ni 3dz2 and 3dx2-y2 interact strongly with ligand lone-pair and σ states. Avoided crossings between ∊(k) curves in k space lead to compositions in the various bands that differ significantly at the bottom and the top. The INDO crystal orbital formalism predicts a partial oxidation of ligand bands in derivatives of 2 that contain oxidants (e.g., halides). The theoretical findings derived for 2 are compared with available experimental data on highly conducting porphyrinatonicke(II) polymers (tetrabenzo and octamethyltetrabenzo derivatives of 2).
    Additional Material: 15 Ill.
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  • 4
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We give an example of multidimensional numerical solutions in momentum space, for the Hartree-Fock equations of the simplest molecule with more than two electrons and two nuclei.
    Additional Material: 2 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 869-879 
    ISSN: 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 aim of this study has been to determine how the hydrogen bond between formaldehyde and ammonia is affected by the ring-opening interaction of protonated ethylene oxide with the ammonia nitrogen. Our purpose is to gain insight into the possible effect of carcinogenic epoxides upon the hydrogen bonding within DNA base pairs; it is known that such epoxides react with nucleophilic sites in DNA, including amine nitrogens that are involved in hydrogen bonds to carbonyl groups. We have used an ab initio self-consistent-field molecular orbital procedure to compute optimized structures, interaction energies, and other properties of interest. The H2CO·HNH2 system and the protonated epoxide were found to form two stable complexes, with syn and anti orientations. Their formation is accompanied by a significant overall strengthening of the H2CO·HNH2 hydrogen bond, as indicated by both calculated force constants and bond lengths. An analysis of these and other properties, such as atomic charges, suggests that the interaction is leading to the formation of N-protonated ethanolamine, which is linked to the formaldehyde through the proton on the nitrogen. If carcinogenic epoxides have a similar strengthening effect upon some of the hydrogen bonds in DNA, it could interfere in the replication and transcription processes.
    Additional Material: 4 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 891-914 
    ISSN: 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 leading term in the asymptotic expansions of the three-center contributions to the exchange matrix elements for a system of three hydrogen atoms is derived. The approximations are used to estimate the importance of three center interactions in “spin-aligned hydrogen”.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984) 
    ISSN: 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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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 915-928 
    ISSN: 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 generalized Hellmann-Feynman and curvature theorems are derived for the generalized Sturm-Liouville-type eigenvalue equation which is widely encountered in many branches of physics. In classical mechanics, the Sturm-Liouville eigenvalue equation primarily arises from applying the theory of small oscillations to vibration problems while, in quantum mechanics, the most important example is the time-independent Schrödinger equation. The generalized theorems, respectively, provide simple useful expressions for the first and second λ derivatives of the eigenvalue where λ is an arbitrary real parameter appearing in the eigenvalue equation. These results, which apply to both classical and quantal systems, include as special cases the well-known quantum mechanical Hellmann-Feynman and curvature theorems for the time-independent Schrödinger equation, thus unifying the formalism. This connection between classical and quantum mechanics is of interest per se, and also because classical eigenvalue equations find application in the treatment of some quantum chemical problems. Two derivations of both generalizations are presented, the first proof applying to exact and Rayleigh-Ritz (linear variational) solutions of the eigenvalue problem, and the second, more general one, to arbitrary optimal variational as well as exact solutions. Necessary and sufficient conditions for the validity of the results are discussed. To demonstrate the insight they afford, several applications of the theorems in conjunction with perturbation theory are described.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 1045-1054 
    ISSN: 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 unitarily invariant decomposition of Hermitian operators for a boson system is performed by means of the symmetry of group SO(2, 1) and the decomposition is also closely related with Lie algebra.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), 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: Methods will be developed for variational calculations of steady-state wave functions and the corresponding quasienergies. These functions appear as solutions to the time-dependent Schrödinger equation for a molecule interacting with an oscillating electromagnetic field. A variation principle is used to obtain coupled equations for each Fourier component (FC) of the steady-state wave function. The FCs are expanded in configuration state functions using the structure of the complete active space (CAS) SCF approach. Two methods are outlined, one which is based on a common set of molecular orbitals for all FCs and one based on an individual optimization of both CI coefficients and molecular orbitals for each FC. The latter method leads to molecular orbital sets for the different FCs which are mutually nonorthogonal. Formulas are derived for the calculation of higher-order dynamic susceptibilities for steady states. An application is made to the H2 molecule, and results are presented for the frequency-dependent polarizability and hyperpolarizability along the molecular axis. The static values obtained are α = 6.413 (6.387) and γ = 676 (674) atomic units, with exact numbers within parentheses.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 91-99 
    ISSN: 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 calculations (self-consistent-field Hartree-Fock) using 6-31G and STO-4G basis sets are used to investigate the interaction between guanidinium and methylguanidinium ion with the carboxylate group of formate. Binding energies and optimum geometries are obtained and compared with reported results using a smaller basis set (STO-3G). The importance of this interaction in proteinsubstrate binding is discussed.
    Additional Material: 2 Ill.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 69-89 
    ISSN: 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: Isomerization of octahedral complexes XY6, by a mechanism known as diagonal twist (in which two ligands in cis positions, one to another, exchange sites) is considered. Construction of the corresponding isomerization graph is outlined (reported before by Balaban) and the problem of determining its symmetry is considered. Alternative routes for deducing the order of the automorphism group are described. The group is confirmed to be S6 and representative symmetry permutations (one for each of the 11 classes of S6) are shown. Alternative pictorial representations of the 15-vertex graph are also shown.
    Additional Material: 12 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 101-123 
    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: This paper lays the algebraic foundation of a unitary group approach to many-body theory. We define a set of second quantized spin-shift operators which are used to construct spin-adapted many-electron configuration functions. We investigate the particle-hole transformation, normal ordering, and contraction of spin shifts. This gives an orbital Wick's theorem reflecting the permutational structure of the states spanning the irreducible representations of the spatial unitary group. We study the spinless Hamiltonian, and the relationship between spin shifts and unitary group generators.
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  • 14
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 151-154 
    ISSN: 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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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 145-149 
    ISSN: 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: Simple density functional theory gives the following relation between the energy EZ, N of an ion of nuclear charge Z and N electrons, the potential V(0) created at the nucleus by the electronic cloud, and the chemical potential μ \documentclass{article}\pagestyle{empty}\begin{document}$$ E_{Z,N} = \frac{3}{7}(ZV(0) + N_\mu). $$\end{document}Using Hartree - Fock values for V(0) and μ, this equation has been tested in several isoelectronic series with 3 ≤ N ≤ 28. The importance of the term 3Nμ/7 increases as the degree of ionization increases.
    Additional Material: 5 Ill.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984) 
    ISSN: 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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  • 17
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 125-143 
    ISSN: 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 paper shows that the spin-shift formalism developed in B. T. Pickup and A. Mukhopadhyay [Int. J. Quantum Chem. 26, 101 (1984)] supports a one-component diagrammatics which has a systematics akin to that in the spin-orbital many-body theory. The diagrams are neither Goldstone nor Yutsis type, and characterize the chain U(2R) ⊃ U(R)⊗SU(2) on which the spin-shift formalism is based. Accordingly, while the lines in such diagrams are labeled by the orbital indices, the diagram structure adequately reflects the irreducible representation of the group U(R). In this sense the paper presents a unitary group approach to the natural generalization of the usual many-body theory for the spin-adapted cases. A set of very simple rules is derived; their similarity with the corresponding rules in the ordinary many-body theory and practical utility are discussed in connection with (a) matrix elements over many-electron spin states and (b) closed- and open-shell many-body perturbation theory. A possibility of integral-driven many-body perturbation theory for open-shells is indicated. Connections of this formalism with others are also discussed.
    Additional Material: 11 Ill.
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  • 18
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 157-166 
    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: The electronic intracule density, a three-dimensional contraction of the spinless electron pair density, is the probability density function for an interelectronic vector. A computationally efficient algorithm for the evaluation of the basic two-electron intracule integral for GTOs is presented. In order to provide an initial understanding of the topography of intracule distributions, anisotropic intracule densities for the X1Σg+ ground states of the H2 and N2 molecules are reported and analyzed.
    Additional Material: 5 Ill.
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  • 19
    Electronic Resource
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 167-181 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With empirical and theoretical atom-atom potentials the GABA·nH2O, n = 25, 192 and GABA·Zn2+ · nH2O, n = 25, 50, 100 complexes are simulated at 298.15 K by the Monte Carlo technique. The results show that the carboxyl group of GABA coordinates six water molecules. Two geometries of the GABA·Zn2+ complex, corresponding to the “direct” and “through-water” interaction of Zn2+ with the carboxyl group of GABA were found. For the latter interaction a GABA·Zn2+ · 6H2O complex was found whereas the hydration of the former interaction leads to a GABA·Zn2+ · 5H2O complex. Here the carboxyl group of GABA displaces only one water molecule in the first hydration shell of Zn2+. Energetically the “direct” and “through-water” geometries seem to be competitive, the former being slightly favored.
    Additional Material: 12 Ill.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 197-212 
    ISSN: 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 Hartree-Fock method devoted to the study of polymers is presented, truncation criteria for the exchange and the Coulomb series are discussed, and it is shown that the error in the total energy evaluation can be reduced to below 0.0001 a.u./cell with little computational effort. Results for six polyacetylene isomers are presented and compared with previous ab initio data obtained using the same basis set. In agreement with the experimental findings, the alternating all-trans structure is the most stable.
    Additional Material: 2 Ill.
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  • 21
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 183-196 
    ISSN: 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 equilibrium geometry of formhydroxamic acid has been calculated within the framework of the INDO-MO formalism. Various structural factors are analyzed and discussed in terms of the calculated force constants and charge distribution. The possibility of internal rotation around the C—N bond of formhydroxamic acid has been examined. The potential energy surface for the amide-imide tautomerism is explored by calculating the geometries and characterizing saddle points on that surface. The cyclic and open dimers of formhydroxamic acid are examined and the hydrogenbond energy and length are calculated.
    Additional Material: 5 Ill.
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  • 22
    ISSN: 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 method for solving correlation pair equations for diatomics is presented and followed by numerical results for the H2 and LiH molecules. The essential feature of the new approach is a numerical representation of both basis two-electron functions and Coulomb and exchange potentials in pair equations. It is shown that this significantly simplifies the pair function optimization procedure.
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  • 23
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 223-230 
    ISSN: 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 calculations are performed with 6-31G basis set to study the geometry and binding of the H3O2-, H5O3-, H7O4-, and H9O5- complexes. The H3O2- complex is also investigated with the 6-31 G* basis set and MP2 (Moller-Plesset perturbation theory of second order).
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  • 24
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    International Journal of Quantum Chemistry 26 (1984), S. 231-236 
    ISSN: 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: Clusters formed by the OH- ion and carbon dioxide are investigated using ab initio Hartree-Fock calculations, with 6-31 G and 6-31 G* basis sets. Geometries and binding energies are determined.
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  • 25
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    International Journal of Quantum Chemistry 26 (1984), S. 237-250 
    ISSN: 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 minimum basis set was developed for use in computing excitation energies of large molecules. It is particularly suited to calculating ionization potentials and Rydberg transitions. Pyrrole excitation energies and oscillator strengths calculated with this basis set are compared to larger basis set ab initio and semiempirical results. The 6-eV band in the experimental spectrum is predicted to be the result of three Rydberg absorptions, with no underlying (π, π*) absorption. The calculations also provide an explanation for the observed N—H stretch in the 6-eV region.
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  • 26
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    International Journal of Quantum Chemistry 26 (1984), S. 293-293 
    ISSN: 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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  • 27
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    International Journal of Quantum Chemistry 26 (1984), S. 463-478 
    ISSN: 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 fairly general degenerate perturbation theory has been considered which involves two versions, i.e., the Rayleigh-Schrodinger and Brillouin-Wigner treatments. The simple recursive formulas have been found for effective Hamiltonians which allow one to split the degenerate energy levels in any orders of this theory.
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  • 28
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    International Journal of Quantum Chemistry 26 (1984), S. 857-872 
    ISSN: 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 suggested that the extreme efficiency and specificity observed for chemical reactions in biological systems (compared to chemical reactions occurring under common chemical conditions) is mainly due to special features of these reactions. A key position is ascribed to the role of weak intermolecular interactions (also called noncovalent or van der Waals); in addition, the important role of the entropy and of cyclic or cavitylike shapes of numerous reaction sites, together with the influence of strong local electric fields on transport phenomena and reactivity are discussed. The greatest attention is paid to the applicability of computational methods for evaluation of weak intermolecular interactions, ranging from beyond Hartree - Fock methods to empirical potentials.
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  • 29
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    International Journal of Quantum Chemistry 26 (1984), S. 943-953 
    ISSN: 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: Enzymes are catalysts occurring in living cells. The aim of this paper is to describe some basic properties of enzymes and to understand on a physical basis how the intracellular milieu may control the kinetics of enzyme reactions. We shall therefore consider in succession some kinetic properties of enzymes in solution and the way cell surfaces and biological membranes control the dynamics of enzyme reactions. The choice of topics which have been selected in this paper is somewhat arbitrary and certainly reflects the tastes and the personal interests of the authors.
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  • 30
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    International Journal of Quantum Chemistry 26 (1984), S. 955-964 
    ISSN: 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 brief review is given of the three main known biochemical mechanisms of human oncogene activation. The underlying possible physical and chemical mechanisms (both short- and long-range) caused by chemical carcinogens are also briefly discussed. The probable role in carcinogenesis of conformational solitons generated after the release of carcinogens previously bound to nucleotide bases is pointed out. For such a soliton the Hamiltonian is written down and the solution of classical equations of motion is outlined.
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  • 31
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    International Journal of Quantum Chemistry 26 (1984), S. 917-931 
    ISSN: 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 arrangement of membrane-bound pigments, proteins, and lipids in the thylakoids of higher plants is described, and the role of the membrane in preventing the back-reaction is discussed. The photosynthetic bacterium halobacterium halobium is also described. A simplified quantum-mechanical picture of the primary process in photosynthesis is presented, and the mechanism of electron-hole separation in this picture is compared with the corresponding mechanism in a silicon solar cell. An appendix discusses the application of Frenkel exciton theory to the antenna effect.
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  • 32
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984) 
    ISSN: 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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  • 33
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    International Journal of Quantum Chemistry 26 (1984), S. 983-985 
    ISSN: 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 proof is given for the triangle inequality of distance function d in the reduced nuclear configuration space M, used in the differentiable manifold model of potential-energy hypersurfaces [P. G. Mezey, Int. J. Quantum Chem. Quantum Chem. Symp. 17, 137 (1983)]. This result completes the proof that M is indeed a metric space with metric d.
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  • 34
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    International Journal of Quantum Chemistry 26 (1984), S. 987-992 
    ISSN: 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 discuss the solution of the integral equation \documentclass{article}\pagestyle{empty}\begin{document}$$ W(x,x') = \upsilon (x,x') + \int {W(x,x''} )P(x'',x''')\upsilon (x''',x')dx''dx''' $$\end{document} over complex domains of integration, such as the constant energy volume in solids and numerically tabulated functions, in the biquadratic limit. This will come in handy in the consistent inclusion of correlation effects in solid-state computations.
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  • 35
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Usefulness of different Gaussian basis sets for reproducing the “tail” region of the SCF wavefunctions employed in calculations of the exchange-repulsion effect is investigated for the model He-He interaction. It has been shown that extension of the monomer-centered basis set in the scheme of regularized even-tempered basis sets [M. W. Schmidt and K. Ruedenberg, J. Chem. Phys. 71, 3951 (1979)] can be more efficient than augmentation of the fully energy-optimized basis set with diffuse basis functions. It has been also found that Landshoff term vanishes and the “tail” region is well reproduced if monomer wavefunctions are calculated with the basis set of the dimer.
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  • 36
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that in the framework of the π-electron approximation even polyenes can be unambiguously divided into 4L- and 4L +2- classes. The classification scheme is based upon the sign alternation of the bond-order between the first and the last atoms and upon the different information content of the bond-order distribution for 4L- and 4L +2- polyenes, respectively. The classification has been shown to hold for the four lowest-lying electronic states: the ground state 1Ag-, the spectroscopically permitted excited state 1Bu+, the spectroscopically forbidden state 1Ag-, and the lowest triplet state 3Bu+ at all levels of sophistication in the π-electron approximation, ranging from the simple Hückel Hamiltonian to the PPP-multi-CI approach. It is shown that the proposed classification is valid also for heteropolyenes and polyenelike/heteropolyenelike fragments from complex organic molecules. The classification has been shown to be useful for the theoretical interpretation of thermo- and photoelectrocyclic reactions. The sign of the bond order between the first and the last atoms (or the average information content) determines unambiguously the topology of the forming cyclic transition state and, thence, the course and the mechanism of these reactions. The results obtained indirectly support the hypothesis that the lowest excited states in butadiene and hexatriene are of the type 1Bu+.
    Additional Material: 4 Ill.
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  • 37
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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  • 38
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    International Journal of Quantum Chemistry 26 (1984), S. 87-89 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problem of code origin is presented in the context of increasingly complex events in the origin of life. The likely sequence of events appears to progress from abiotic synthesis of biological monomers to polymers to formation of protocells, which would be capable of competition and further evolution. We propose that rate of polymer formation was a critical controlling parameter of rate of protocell propagation. This would lead to selection of autocatalytic and mutually catalytic reactions of polymer formation. Primitive proteins would catalyze polynucleotide formation, and polynucleotides could be used as anvils of noncoded polypeptide synthesis. Proteins that could catalyze this latter reaction (assignment catalysts) would play an important role in subsequent evolution of a genetic code. Competing populations of assignment catalysts would possess very nonlinear dynamics of production of the catalysts themselves. An analysis of this dynamics shows that it has a rich family of bifurications which would provide a pathway for gradual approach to a genetic code. The selection criterion in this process would be efficiency of utilization of monomers and energy for the production of assignment catalysts.
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  • 39
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    International Journal of Quantum Chemistry 26 (1984), S. 167-181 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations have been carried out on the reaction of NO+ with amines, using a 4-21G basis set. The influence of solvation was investigated using one to three molecules of H2O. Geometry optimizations were carried out on reactants, products, and intermediates. The results show that loss of CH3+ is energetically favorable and this fact has implications with respect to the mechanism of carcinogenesis by dimethylnitrosamine.
    Additional Material: 6 Ill.
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  • 40
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Activation of amino acids appears to have played a crucial role in prebiotic peptide bond formation. As a model of this process in living systems, phosphates have been used as amino acid activators. The possible role of clay and other minerals has also been investigated. We are presently using ab initio methods to investigate the activation of amino acids by these agents, as an initial step in peptide bond formation. A model of this activation process is described by the reaction: The first step in such an investigation, reported here, was to determine the lowest energy structures of a suitable set of reactants. As initial models of amino acids, Z was chosen to be H and NH2, corresponding to acetic acid and glycine, respectively. XO4Hn+1 = H3PO4 represents a phosphate group, while Si(OH)4 describes an edge tetrahedral site of a clay mineral. Al(OH)4- was also included to represent a tetrahedral edge site where the silicon is replaced by an aluminum. Finally, to complete the series XO4Hn+1, H2SO4 was added to the set of reactants. All species were optimized using the STO-3G and STO-3G* basis sets. For H3PO4 and Al(OH)4-, STO-3G* full optimizations were not possible. In these cases, certain torsional angles were optimized separately, then held at the optimized value, while the rest of the bond lengths and angles were optimized. All structures were compared to other calculations and to experimental geometries when available.
    Additional Material: 7 Ill.
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  • 41
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    International Journal of Quantum Chemistry 26 (1984), S. 155-166 
    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: This article summarizes and extends our computational studies (ab initio, SCF-MO) of the reactive properties of halogenated hydrocarbon epoxides. For five such epoxides (ethylene oxide, propylene oxide, chlorooxirane, trans-dichlorooxirane, and epichlorohydrin), we analyze and compare first the energy requirements for stretching the C—O and C—Cl bonds, and second, the reactivities of the epoxide ring carbons toward a model nucleophile, ammonia. At each step along the various reaction pathways, the structure of the system was reoptimized. The epoxides were taken to be protonated, either on the oxygen or on the chlorine. Ring opening via monomolecular rupture of a C—O bond was found to occur significantly more readily when there is a —CH3 or —Cl substituent on the carbon. Epichlorohydrin is exceptional, in that stretching a C—O bond leads to a movement of the chlorine toward the carbon in question, forming a three- or four-membered ring. The stretching of protonated C—Cl bonds has remarkably low energy requirements, even when the carbon is not part of the epoxide ring. The interactions with ammonia produced intermediate complexes, which are particularly stable when there is a chlorine on the other ring carbon. The formation of the primary in vivo DNA alkylation product of vinyl chloride, suggested as being responsible for the carcinogenicity of the latter, is discussed. The most negative values of the electrostatic potentials near the oxygens of 21 different epoxides are listed and analyzed in terms of their relationship to the nature of the substituent on the epoxide ring. Also discussed are our earlier findings that epoxide carcinogenicity appears to be associated with a relatively strong negative potential near the oxygen, and that the abilities of epoxides to inhibit epoxide hydrase correlate well with this oxygen potential (modified by a parameter to take account of steric effects).
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  • 42
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    International Journal of Quantum Chemistry 26 (1984), S. 195-200 
    ISSN: 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 previous research from these laboratories various structural analogs of 3-hydroxyflavone were tested for inhibition of glyoxalse I (S-lactoyl-glutathione methylglyoxal lyase, isomerizing; EC 4.4.1.5). The substrate of glyoxalse I (Glo I), methylglyoxal, has growth inhibitory properties. Glo I was purified 7000-fold from human red blood cells, and the concentration of various flavones was determined for 50% inhibition (I50) of enzyme activity. These compounds resemble the transition state of the methylglyoxal hemimercaptal as previously reported [Int. J. Quantum Chem. Quantum Biol. Symp., 10, 357 (1983)]. The I50 in μM varies from 5 to 330 for the flavones tested, with the parent compound 3-hydroxyflavone having an I50 of 10 μM. The most inhibitory compound in vitro was myricetin (5 μM), which has a 3′,4′,5′-trihydroxyphenolic ring at the 2 position of the pyrone ring.
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  • 43
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    International Journal of Quantum Chemistry 26 (1984), S. 209-216 
    ISSN: 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 transport of deuterium ions in α-cyclodextrin has been monitored using a mass spectrometer to show how the protonic conductivity varies with the extent of water-based hydrogen bond networks in the cyclodextrin hydrate. By means of mass-spectrometric and microprobe analysis measurements it has been possible to distinguish between ionic and protonic conductivity in hydrated lysozyme, and further details have been obtained concerning the physical origins of the low frequency dielectric dispersions (the so-called α and Ω dispersions) exhibited by hydrated protein powders.
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  • 44
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    International Journal of Quantum Chemistry 26 (1984), S. 201-208 
    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: Various configurations of the system composed of benzene and acetylene are investigated in an effort to identify the most stable. The total interaction energy is computed as the sum of the dispersion energy ED and ΔESCF, the interaction energy calculated by ab initio molecular orbital methods. Pairwise summation schemes of both the atom-atom and bond-bond types are used to evaluate ED. The most stable structure is found to be that in which the acetylene approaches benzene from above and is oriented such that it lies directly along the C6 axis of benzene. Although the contribution of dispersion to the total interaction energy is dominant, the smaller electrostatic component plays a crucial role in controlling the relative orientations of the two molecules. Indeed, it is possible to select the most stable configuration solely on grounds of the most favorable quadrupole-quadrupole interaction. The likelihood of observation of this configuration is further confirmed by consideration of basis set extension and entropic effects.
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  • 45
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Experimental studies on 5-hydroxytryptamine (5-HT) and its congeners have shown these compounds to interact with the same receptors in peripheral tissues and in brain. To evaluate the importance of the relative position of the 5-HT-like recognition elements at these receptors, we studied two compounds structurally related to 5-HT in which the structural elements involved in receptor recognition are positioned differently from 5-HT: 5-hydroxyaminotetrahydrobenzindole (FHATHBIN) in which the position of the side chain is fixed with respect to the indole, and 4(β-aminoethyl)-5-hydroxyindole (FAEFHI) in which the side chain is flexible, and connected to the indole at the C4 position (rather than at C3 as in 5-HT). Ab initio molecular orbital calculations of the molecules and model fragments were performed with STO-3G and 3-21G basis sets, using structural optimization procedures. The results show that both structures possess the reactivity elements required for the interaction with the 5-HT receptor, but that FAEFHI cannot be recognized at the 5-HT receptor because the side chain is held in the wrong conformation with respect to the indole portion by a strong hydrogen bond between the side chain amine group and the hydroxyl at C5. We report results from competition experiments for binding at high affinity 5-HT binding sites in brain membranes which support this conclusion by showing that FAEFHI has low affinity for these sites.
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  • 46
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    International Journal of Quantum Chemistry 26 (1984), S. 217-222 
    ISSN: 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 proton hyperfine coupling constants for the family of methoxyl-substituted semiquinones in aqueous solution have been determined by electron spin resonance (ESR) spectroscopy. These data and the electrochemical midpoint potentials for the corresponding series of substituted quinone/hydroquinone couples are discussed. Neither the additivity principle describing quinoidal hyperfine coupling constants nor the Hammett substituent relationship predicting electrochemical behavior are obeyed by these methoxyl-substituted quinones. It is shown, however, that the experimental data can be rationalized in terms of neighboring group interactions that occur between adjacent methoxyl substituents. Such perturbations apparently lead to a breakdown of hyperconjugation between the electronic bonding structure of the quinone ring and the methyl group of the methoxyl moiety. It is pointed out that ubiquinone is a dimethoxyl compound whose properties are determined by such neighboring group interactions. This observation may have important consquences for the fabrication of ubiquinone analogs and quinone-based drugs.
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  • 47
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    International Journal of Quantum Chemistry 26 (1984), S. 223-235 
    ISSN: 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 Slater-Koster resolvent formalism of exciton theory, as proposed originally by Takeuti, has been applied to calculate charge transfer exciton states and to investigate hypochromism in polynucleotides. As a first step, spatially well localized ab initio Wannier functions (WFS) are calculated at the Hartree-Fock level using a two-phase (inter- and intramolecular) localization procedure for the Fourier transformation of the Bloch functions. The single particle energies, entering the Green's function of the polymer, are corrected for electron correlation effects with the help of second order Møller-Plesset (MP) perturbation theory. The interelectronic matrix elements, used in the MP calculation as well as in solving the resolvent problem for the excitons, are calculated in terms of the WFS. Singlet- and triplet-excitonic dispersions, oscillator strengths, the possible affects of ions, hydration, and aperiodicity on the exciton spectrum are discussed.
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  • 48
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ability of three cultured cell lines (NRK, 6M2, and 54-5A4) derived from rat kidney to quench a population of ascorbyl and 2,6-dimethoxy-p-semiquinone free radicals has been investigated by electron spin resonance spectroscopy. The radical scavenging action of normal rat kidney (NRK) cells was the weakest, that of 6M2 cells (reversibly transformed phenotype) was four times stronger than NRK, and that of 54-5A4 cells (irreversibly transformed mutants of 6M2) was 10 times greater than NRK. Free radical quenching experiments were also performed on Chinese hamster ovary (CHO) cells. A much slower scavenging rate was observed for CHO cells of normal phenotype grown in the presence of dibutyryl cyclic AMP than was found for cells of transformed phenotype grown in its absence. The free radical quenching kinetics of the various cell lines studied directly paralleled their state of transformation.
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  • 49
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Single-channel studies have shown that there exist a multiplicity of conducting states in Gramicidin A (GA). In an earlier work presented at this symposium, it was proposed that such a multiplicity may be expected from different long-lived side-chain distributions available for the channel molecule. In order to test this hypothesis, Leu5-Gramicidin A was synthesized and the effect of replacing the L · Ala5 residue by L · Leu5 was analyzed. First, molecular mechanics calculations on Leu5-Gramicidin A are presented. Then the single-channel conductance sweeps obtained for Leu5-GA are automatically analyzed in the computer using specifically developed algorithms. The results show a dramatic decrease in the multiplicity of states due to the Leu5 substitution. This experimental result is discussed in the light of structural concepts emerging from molecular mechanics calculations.
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  • 50
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    International Journal of Quantum Chemistry 26 (1984), S. 347-353 
    ISSN: 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 Hamiltonian formulation of chemical kinetics is applied to all closed, single-step, integer-order chemical reactions. In all cases, the dynamics of the Hamiltonian leads to the correct phenomenological rate equation. The relationship between reaction order and the form of the reaction potential is discussed.
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  • 51
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    International Journal of Quantum Chemistry 26 (1984) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 52
    ISSN: 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: Pb2+ specifically cleaves yeast tRNAPhe in orthorhombic crystals at phosphates P18 and P60 in the dihydrouridine (D) and pseudouridine (T) loops, respectively. Although there are three major lead binding sites in the tRNA, it appears that both cleavages are affected by the same lead ion. The P18 site is apparently the same site that is hydrolyzed by Pb2+ in solutions of the tRNA and is the major cleavage site in crystals. It is shown from P32 labeling studies with polynucleotide kinase that the products of Pb2+ cleavage have the 5′-hydroxyls on G18 and C60, and consequently the 3′ phosphates are on D17 and U59. The OH- in the first coordination sphere of the hydroxo complex Pb(OH)+ either directly or through a water molecule abstracts the proton of the 2′-OH of D17. This promotes a nucleophilic attack by 2′-O- on P18 with subsequent cleavage of the P—O5′ ester bond to generate the major Pb2+ cleavage product pG18-A76. The anchoring of the Pb2+ by the T-loop and D-loop residues seemingly provide the right environment for the generation of the hydroxo complex required for the hydrolysis. Additionally the strain of the phosphodiester conformation at P18 enhances the cleavage at that site. The Pb2+-catalyzed cleavage of tRNA exemplifies a mechanism used not only for the hydrolysis of transfer RNA but one potentially used by other RNA-catalyzed reaction utilizing metal ion cofactors.
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  • 53
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    International Journal of Quantum Chemistry 26 (1984), S. 33-41 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We discuss in this paper the various objections to the Kubo formalism initiated by van Kampen. We motivate the need to embed the system of interest in a larger reservoir to resolve these objections using the van Hove limit. In particular, in the context of spin magnetic systems, we show that it is precisely the spin-lattice coupling together with the above limit which is responsible for the validity of the linearization ansatz.
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  • 54
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    International Journal of Quantum Chemistry 26 (1984), S. 57-60 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A second-quantization formalism is used to derive closed formulas for one- and two-center matrix elements of two harmonic oscillators with different force constants. The method is based on a linear transformation between the creation and annihilation operators of the two harmonic oscillators, the use of the Baker-Campbell-Hausdorff theorem, and Cauchy's integral formula. As presented, the proposed procedure is considerably simpler than the ones hitherto employed.
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  • 55
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    International Journal of Quantum Chemistry 26 (1984), S. 61-66 
    ISSN: 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 use of computer algebra and expansions of orbitals about a single center in spherical harmonics with a matrix representation for the α-function coefficients it is possible to produce formulas for each term of an infinite series that represents the value of a three-center exchange integral. Only seven terms are required to get six decimal digits for triatomic hydrogen. This method is readily generalized to solve all three-center exchange integrals over Slater-type orbitals.
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  • 56
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    International Journal of Quantum Chemistry 26 (1984), S. 43-56 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Canonical generator states provide an overcomplete, nonorthonormal basis for the irreducible representation spaces of the unitary group. The matrix representation of any unitary-group operator in the generator basis can be directly computed by Lie algebra techniques and can be converted, if desired, to a representation over Gel'fand states by inverting the Moshinsky-Nagel transformation. Application is made to the symmetry adaptation of the Hubbard allyl radical with respect to reflection, guasispin, and R(3) symmetries.
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  • 57
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    International Journal of Quantum Chemistry 26 (1984), S. 67-76 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: As a first step toward a solution of the general Hartree-Fock equations for a one-dimensional Coulomb system we construct charge current waves by pairing restricted Hartree-Fock orbitals with functions from their orthogonal complement. The degree of pairing is determined by the solution of the corresponding gap equation. Special attention is paid to the screening problem.
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  • 58
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    International Journal of Quantum Chemistry 26 (1984), S. 121-130 
    ISSN: 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 be useful for ionic crystals cluster calculations must be modified so as to simulate the crystal potential in a reasonable way. Two aspects of such calculations - the normalization of the orbitals and the construction of the coulomb potential - are analyzed in some detail. Illustrative calculations are carried out for MgO and the results are compared with experimental data and with other theoretical results.
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  • 59
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    International Journal of Quantum Chemistry 26 (1984), S. 131-140 
    ISSN: 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: Dissociation reactions of NaCl into the ionic and the neutral atomic states and the interactions Na+-Na+ and Cl--Cl- of the like ions are investigated quantum mechanically at the configuration interaction level of approximation. Data including the calculated total equilibrium energies, dissociation energies, and bond distances are reported together with the tabulated interaction energies at a wide range of nuclear separations.
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  • 60
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    International Journal of Quantum Chemistry 26 (1984), S. 141-152 
    ISSN: 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 analysis is made of the mathematical and computational implications of a recently proposed scheme to calculate the electronic structure of chainlike systems without explicit use of the LCAO density matrix elements. The infinite chain of hydrogen atoms is chosen as a simple test case illustrating the underlying difficulties associated with the scheme. It turns out that the positive features that have motivated the development of this approach do not balance the numerical difficulties. It seems unlikely that, in routine applications, the new strategy will become a real competitor to the existing programs.
    Additional Material: 1 Ill.
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  • 61
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 153-159 
    ISSN: 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 crystal orbital calculations with extended basis sets and model potentials are performed for one-dimensional periodic systems. The results of restricted Hartree-Fock-type calculation for all-trans polyacetylene (CH)n and linear and bent hydrogen-bonded poly(hydrogen fluoride) (HF)n are mainly reported. The results, in particular total energies per unit cell, are shown to be much more dependent upon the basis sets than on the method of lattice-sum truncation, number of k points, or order of neighbor interactions.
    Additional Material: 1 Ill.
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  • 62
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), 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: Møller-Plesset perturbation theory has been applied to investigate the influence of electron correlation on the mechanical and optical properties of infinite periodic polyethylene (PE) and on the optical properties of polydiacetylenes (PDAs). The microscopic Young's modulus and deviations from Hooke's law have been calculated for PE using both restricted and unrestricted Hartree-Fock (RHF, UHF) zeroth-order wave functions. These deviations become really important in the region of 30-40% elongation ∊ and they lead to a maximum of the retractive force of f = 0.814 mdyn at ∊ = 0.38 and of f = 0.631 mdyn at ∊ = 0.31 in the case of RHF and UHF reference functions, respectively. The best theoretical value of the longitudinal elastic modulus of PE (comparing different atomic basis function sets) is 334 GPa at the HF level and 276 GPa including electron correlation. The charge transfer exciton spectrum of PE and of PDA models has been calculated with the help of a Green's-function approach using quasiparticle states of the electron polaron method. The first singlet excitations occur at 8.67 eV in PE and at 1.7-2.1 eV in PDAs, in good agreement with experiments.
    Additional Material: 3 Ill.
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  • 63
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 183-193 
    ISSN: 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: Aspects of recent local density (LD) studies on transition-metal clusters, surface complexes, and oxide defect structures are presented. The evolution of the LD theory as a reliable means of predicting spectroscopic and structural properties of transition-metal systems is explored. As specific examples we consider the magnetic impurity problem, the chemisorption of simple molecules, and the effect of oxide vacancies on x-ray absorption cross sections.
    Additional Material: 5 Ill.
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  • 64
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 173-182 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of self-consistent field-local spin density-scattered-wave calcualtions are reported for small cluster (13-14 atoms) containing nickel and copper atoms in various proportions and for these clusters interacting with a CO molecule adsorbed on a top or a hollow site. The effect of alloying on the magnetism of the clusters in similiar to that found in the bulk case, namely (i) the Ni magnetic moments decrease with Cu content and (ii) the spin polarization on the copper atoms is very small. The effect of Cu is mainly limited to the fist nearest-neighbot Ni atoms. In agreement with experiment, adsorption of CO on pure Ni clusters causes a decerese in the magnetic moments of nearby Ni atoms, and we have recently proposed a detailed mechanism which explains this effect. The influence of Co on Ni magnetism for Ni—Cu + CO clusters is investigated and is shown to be the same as in the pure Ni case as long as no Cu atoms are present in the first coordination shell of the Ni atom(s) involved in the bond. If Cu atoms are present in the first coordination shell, the stabilization of the Ni potential by CO chemisorption leads to increased Ni—Cu covalent interactions. For the cases we have examined, this modified Ni—Cu binding leads to a small increase rather than the usual decrease of the Ni moment for the atom directly involved in the chemisorption.
    Additional Material: 5 Ill.
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  • 65
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 195-211 
    ISSN: 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: Current experimental efforts, both with isolated binuclear metal complexes and with modified proteins, are focusing on the role of distance and of bridging groups in intramolecular electron transfer. After a brief overview both of standard and nonadiabatic electron transfer rates (in Hopfield's formulation) and of the current experiments. We consider the possiblility of bridge-assisted intramolecular electron transfer in three specific situations. For the bridged binuclear transition-metal complexes studied by Taube and his students, the tunneling integral T can be deduced by study of the optical intervalence transfer band. Here we discuss semiempirical calculations which suggest that bridge assistance occurs, and determine the size of T. The assistance is via a superexchange-type mechanism, and we suggest that hole-type or electron-type superexchange should dominate in saturated or unsaturated bridges, respectively. For the very strongly coupled Creutz-Taube ion, involving a pyrazine bridge, the electronic structure study of Ondrechen et al. requires the invoking of a three-site model to understand the optical and EPR data; this involves bridge-assisted transfer with a vengeance. Finally, for an oxide-bridged phthalocyanine dimer, the transfer takes place via ring π-π overlap, and no bridge assistance occurs. Thus bridge assistance in T will depend on the geometry and energetics of the specific case under study. One generally suspects, however, that bridge assistance will be found in very long-range (〉6 Å) transfer.
    Additional Material: 4 Ill.
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  • 66
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 237-244 
    ISSN: 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: Problems which arise in the application of closed-shell coupled-cluster approaches to quasidegenerate or almost degenerate situations are discussed and the basic classification of quasidegeneracy types is outlined. Recent coupled-cluster results obtained for the cyclic polyene model, particularly in the strongly correlated limit, are briefly discussed and the unexpected features of approximate and localized coupled-pair approaches are pointed out.
    Additional Material: 1 Tab.
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  • 67
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 245-254 
    ISSN: 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 nondiagrammatic formulation of the analytical first derivative of the coupled-cluster (CC) energy with respect to nuclear position is presented and some features of an efficient computational method to calculate this derivative are described. Since neither the orbitals nor the configuration expansion coefficients are variationally determined, in the most general case derivatives of both are necessary in computing the gradient. This requires the initial solution of the coupled perturbed Hartree-Forck (CPHF) equations and seems to mandate the solution of a linear matrix equation ZT(1) = X for first-order corrections to the CC coefficients. However, if only the analytic gradient is desired a simpler non-perturbation-dependent set of equations can be solved instead. This and the first-order character of the linear matrix equation makes the application of an analytic gradient technique to the CC method feasible.
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  • 68
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    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 255-265 
    ISSN: 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: Expressions for static and dynamic properties in coupled-cluster (CC) theory are derived. In the static case, using diagrammatic techniques, it is shown how consideration of orbital relaxation effects in the theory introduces higher-order correlation effects. For the dynamic case, excitation energy expressions are obtained without consideration of orbital relaxation effects and shown to be equivalent to an equation of motion (EOM) approach subject to a coupled-cluster ground-state wave function and an excitation operator consisting of single and double excitations. Illustrative applications for excited states of ethylene are reported.
    Additional Material: 4 Ill.
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  • 69
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    International Journal of Quantum Chemistry 26 (1984), S. 293-307 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics