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  • Atomic, Molecular and Optical Physics  (269)
  • 1990-1994  (269)
  • 1970-1974
  • 1965-1969
  • 1990  (269)
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  • 1990-1994  (269)
  • 1970-1974
  • 1965-1969
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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: Partial cross sections of single-electron capture on the n = 3 levels have been determined theoretically for the N5+ + He and O6+ + He collisions by means of a semiclassical method using ab initio potential energy curves and radial and rotational coupling matrix elements. The different behavior of these two isoelectronic systems is fairly well reproduced by our calculations.
    Additional Material: 5 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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: High-quality ab initio calculations, i.e., calculations including electron correlations with large atomic basis sets, have been restricted to relatively small systems due to their excessive demand on computer facilities. To extend these methods to large systems, one is faced with two problems: the steep (6 power) increase of the computational task with the molecular size, and the large number of electron repulsion integrals that must be stored and retrieved. We have recently introduced new computational methods where both these problems have been addressed. The local correlation method reduces the steep dependency of computational effort with molecular size without significant loss of accuracy by taking advantage of the localizability of the electronic structure. For electron correlation calculations integrals in the MO basis are needed in addition to the two electron integrals in the AO basis, and the only way electron correlation can be applicable to large systems is by a direct approach in the spirit of Almlof's direct SCF. We have recently extended the direct approach to post Hartree-Fock methods. Description of both these methods including their supercomputer implementations are discussed.
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  • 3
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new formalism has been developed in order to evaluate intermolecular interaction energies for organometallic complexes including electrostatic, polarization, and orbital contributions based on extended Hückel molecular orbital (EHMO) theory. The electrostatic interaction is evaluated using (i) a multipolar expansion of EHMO charge density, or (ii) by calculating directly the electrostatic integrals in the basis of atomic orbitals. The polarization effects are evaluated by introducing a perturbation into the Hamiltonian. The orbital interaction is calculated by considering a supermolecule made of the organometallic substrate and a model electrophile or nucleophile. To provide the shortest possible response time on an interactive computer graphics facility, this model should require the minimum amount of computer time, which explains why approximate procedures are used to evaluate the dominant contributions to the interaction energies. Preliminary results show that these interaction energies lead to reaction potentials in good agreement with experiment for a broad series of nucleophilic and electrophilic addition or substitution reactions involving organometallic complexes. In addition, it is shown that the method can easily be extended for the calculation of solvent effects. To this end, developments considering the supermolecule surrounded by a polarizable continuum are in progress.
    Additional Material: 9 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: Five energy hypersurfaces of the most examined pericyclic reactions have been investigated by using ab initio SCF, CASSCF, and the semiempirical AM1 methods. The systems are H4, H6, C3H6, C4H4, and C3OH4. Stationary points and their sets of harmonic vibrational frequencies have been calculated by means of analytical gradient techniques in the frameworks of the respective approaches. ZPE corrected energy barriers are based on single-point calculations including dynamical correlation corrections by CAS-CI(SD)+DC, CASCEPA, or MP2.
    Additional Material: 6 Tab.
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  • 5
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 intermolecular interaction between the molecules CH2O and NH3 was investigated by the supermolecule method. The interaction energies were first calculated at the ab initio SCF level, and the electron correlation was included via second-order Møller-Plesset perturbation theory (MP2). The basis set superposition error (BSSE) was taken into account by the counter-poise (CP) method. The occupied and the virtual canonical molecular orbitals (CMOS) of the supermolecule were separately localized by the Boys' procedure. The correlation correction was calculated by the many-body perturbation theory (MBPT) in the localized representation. Contributions of the third- and fourth-order localized diagrams were added to those of the second-order canonical diagram. This procedure gives a correction nearly equivalent to that of MP2. The possibility to separate LMO contributions responsible for the dispersion interaction was investigated.
    Additional Material: 8 Tab.
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  • 6
    ISSN: 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 quantum field mechanics of an electron subsystem in 3D physical space as the topology of compact atomic clusters with spontaneously broken local canonical symmetry is used for investigation of different types of microdefects in the condensed state of transition metals. The theory is illustrated with results of calculation of small compact Fen clusters (n = 2, 6, 14).
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  • 7
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 method is presented for obtaining rapidly convergent upper and lower bounds to the eigenvalues of the Schrödinger equation for one-dimensional and central-field models. The logarithmic derivative of the wave function is written as a Padé approximant and the bounds are obtained by simply counting the real zeroes of the denominator.
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  • 8
    ISSN: 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.
    Additional Material: 8 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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: Explicit equations for the constants a and b of the energy as the average expectation value for the minimal domain of states giving the Roothaan energy functional are derived in terms of the total spin value and the “seniority” quantum number. The state with off-diagonal long-range order can appear in the framework of the Roothaan scheme for the quasidegenerate system as an alternative to the Hund rule. Admissible many-electron states are established for all configurations of the icosahedral symmetry group systems, and corresponding a and b coefficients are calculated.
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  • 10
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 explains a method for finding saddle points on a multidimensional surface and shows how it may be used to define saddle-point seeking curves that have properties similar to well-known orthogonal trajectories. It is shown that a gradient extremal is a special case of one of these curves, and its chemical significance as the path, defined by local criteria, which starts from a stable structure and leads to a transition state, is discussed briefly in relation to the intrinsic reaction coordinate. It is emphasized that this theory gives a natural method for locating points that have Hessians of similar structure to those of transition states.
    Additional Material: 3 Ill.
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  • 11
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 correlation potential is computed for two electron atomic ions with atomic numbers from 1 to 10 using the charge density reconstructed from a natural orbital expansion of a Kinoshita-like atomic wave function. Over the wide range of densities involved, the correlation potentials are not even approximately a local function of the density.
    Additional Material: 4 Ill.
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  • 12
    ISSN: 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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  • 13
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 16
    ISSN: 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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  • 17
    ISSN: 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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  • 18
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Based on the completely optimized S0, S1, and S2 molecular geometries of azulene, the vibronic structure of the S1 ← S0 absorption as well as of the S1 → S0, S2 → S1, and S2 → S0 fluorescences is investigated theoretically within the adiabatic approximation. By means of theory-experiment comparisons, the influence of non-Condon terms and of the Dushinsky effect on the vibronic structure of azulene spectral behavior is discussed. Typical for the S1 ← S0 absorption and the S1 → S0 fluorescence are vibronic transition moment contributions of Condon type, whereas the interpretation of azulene S2 → S1 and S2 → S0 fluorescences is successful only within the scope of the Herzberg-Teller approach by taking into account vibronic coupling terms and, additionally, the Dushinsky effect in the latter case. An analysis of the relevant vibrational modes is given.
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  • 19
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, the symmetrized basis function method is extended and used for the classification of the total spin states of many-electron systems. The reduction matrix, which is expressed as a series of products and direct products of matrices is derived. It is very advantageous that the method can be completed by computer. As an example, the reduction matrix for a 5-electron system calculated by IBM PC computer and the classification of the total spin states is presented.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this paper, the TST-CEQ calculated results including the reactive cross sections and rate constants for the Cl + HCl reaction at the specified vibrational state of the HCl are presented. It is shown that the oscillatory behavior of the system in the three-dimensional world does not appear, and the larger the vibrational quantum number of HCl, the smaller the cross sections ¯QTST-CEQ (E, ν) are at a given total energy used in the calculations, but the larger the reactive rate, constants KTST-CEQ (T, ν) are at a given temperature. The KTST-CEQ (T, v) is the same as when first published, and the average reactive rate constants KTST-CEQ (T) are close to the ones by Garrett and Schatz et al.
    Additional Material: 4 Ill.
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  • 21
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A pair-excitation-multiconfigurational-self-consistent-field (PEMCSCF) study with 10 configurations has been carried out on 3-methyl-1,2,4-oxadiazolin-5-one (3-methyl pseudosydnone) which has normal covalent bonds but is isoelectronic with 3-methyl sydnone for which no single covalent valence bond structure can be drawn. The PEMCSCF results were found to be very similar for the two isoelectronic compounds and an attempt was made to represent the bonding of the sydnone in only two simple molecular-orbital-valence-bond (MOVB) configurations using a Luken-type correlating orbital. A compact minimum basis of Slater-Transform-Preuss functions fitted by six cartesian gaussian orbitals (STP-6G) is used to express the compact representation for chemical comparison to experiment. The molecular geometry was gradient-optimized at the single determinantal level using a 6-31G extended basis set. “Orthonormal Resonance Analysis” is given for the two dominant configurations by noting that the Boys-Reeves configuration interaction algorithm is identical to the Pauling valence bond algorithm except for use of orthogonal molecular orbitals instead of atomic hybrid orbitals. This equivalence permits use of PEMCSCF atomic charges, bond orders, and dipole moments in a slightly modified valence bond interpretation of the unusual bonding of this compound. Although forcing a larger PEMCSCF problem into a minimum basis and only two configurations raises the energy, chemical interpretation is simplified. This comparison between a mesoionic compound and an isoelectric compound with normal bonding offers new understanding of the mesoionic effect.
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  • 22
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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.
    Additional Material: 3 Ill.
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  • 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: The formalism of Colle and Salvetti for the calculation of the correlation energy of electronic systems has proven to be highly successful in a wide variety of applications. Several authors have applied the method and analyzed its usefulness.The Colle and Salvetti ansatz for the correlation energy isWe test the limits of validity for expression (2) by applying it to the series of ions isoelectronic with helium, (two-electron case) and beryllium, (four-electron case). We find that the approximation breaks down for sufficiently high Z. We also compare the results given by Eqs. (1) and (2). We find that although both expressions give the same result for the neutral He atom, in general they do not.
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  • 24
    ISSN: 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 review the basic principles of the various coupled cluster (CC) methods based on an exponential form for the many-body wavefunction, and contrast them with the configuration-interaction (CI) method. Particular emphasis is placed on their applicability to problems in quantum chemistry. We prove that in all cases we can construct an energy functional which variationally determines both the ground-state wavefunction and the dynamic equations of motion for nonstationary states. As a result the equations of motion assume the familiar classical canonical Hamiltonian form in some well-defined (multibody) configuration space. We also thereby construct the expectation-value functional for an arbitrary operator in such a way that the Feynman-Hellmann theorem is preserved at all natural levels of truncation of the appropriate configuration space. We show in detail that only in the case of the recently introduced extended CC method (ECCM) is the expectation-value functional expressed fully in terms of linked (multilocal) amplitudes. The ECCM is thereby capable of describing such global phenomena as shape transitions and other stereochemical properties, and the large-scale behavior of the molecular energy surfaces. We illustrate our methodology on the one-body density matrix, which is now much more easily discussed than by conventional methods in quantum chemistry.
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  • 25
    ISSN: 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: Relativistic coupled cluster (CC) calculations are performed for the atomic systems Be, Ne(6+), Ar(14+), and Sn(46+). The CC equations through CCSDT are solved within a space spanned by balanced s1/2 gaussian spinors to obtain the s-correlation contributions. No significant contribution of higher correlation effects beyond MBPT (4) is observed within this s1/2-spinor approximation.
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  • 26
    ISSN: 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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  • 27
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of pure water are reported. Ewald summation is found to retard translational diffusion. Results are compared for several simulations using different boundary conditions.
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  • 28
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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: Moments present a convenient mean to describe molecular connectivity. The method of moments has been used to evaluate the total pi-electron energy Epi, the atomic charge Qr, and the free valence index Fr, in conjugated molecules. Because of practical reasons, only the moments up to some finite order L can be considered. Generally, the coefficients of the expansion of a property in terms of moments depend on L. In order to avoid this dependence, the expansion in terms of orthogonal polynomials has been used. Validity and convergence properties of such a method has been examined. The method has been also applied to some conjugated polymers.
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  • 29
    ISSN: 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 algorithm for locating stationary points corresponding to local minima and transition states on potential energy surfaces is further analyzed. This method utilizes local gradient and Hessian (i.e., first and second energy derivative) information to generate a series of “steps” that are followed to the desired stationary point. By designing the step sequence to move energetically downhill in all coordinates, local minima can be found. By stepping uphill along one local eigenmode of the Hessian while minimizing the energy along all other modes, one locates transition states. Key elements of this development are more efficient parameterization of the step vector in terms of quantities that permit the direction (i.e., uphill or downhill), and length of the step to be carefully controlled, and implementation of the ability to explore “side channels” as attractive options occur.
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  • 30
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New experimental techniques combined with computer molecular dynamics simulations of increasing complexity are beginning to permit a detailed examination of the initial steps in the complex chemistry describing the high-temperature, high-pressure regime associated with combustion and detonation as well as processes that may be involved in shock-induced chemistry. We present the results of several simulations that describe the generation of moderately strong shocks and their interaction with molecules embedded in weakly-bound clusters, with material defects such as microscopic voids, and with molecular species placed in the neighborhood of such defects.
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  • 31
    ISSN: 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 coupled-cluster method with single and double excitations (CCSD) is used to investigate the equilibrium configuration and vibrational frequencies of N3 and N-3. Good agreement with all experimental spectroscopic constants is obtained. Both species are found to have D∞h minima, with Re = 1.190A for the anion (experimental 1.1884A) and 1.183A for the radical (exp. 1.18115A). Vibrational frequencies are accurate to 20-40 cm-1. The calculated electron affinity of N3 is 2.50 eV.
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  • 32
    ISSN: 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 simple nonempirical scheme is presented for calculating the splittings of ground state multiplets (the zero-field splitting) in transition metal complexes. The method employs single reference, single excitation CI calculations based on open-shell RHF. The spin-orbit coupling is described using an effective one-electron, one-center operators. The method is applied to the triplet state Ni(II) complexes with one to six water molecules. The validity of the second-order perturbation theory approach and of the spin-Hamiltonian formalism is found to be limited to slightly distorted octahedral systems. Generally, small changes in the geometries of the complexes are found to cause substantial variations of the splitting pattern.
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  • 33
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A molecule whose electronic structures are poorly understood, in some cases, can be thought of as another perhaps well-understood “host chemical molecule” with substitutional “impurities.” From this point of view, the Green's function technique in solid state physics can be extended to investigate the change due to substitutional impurities in large chemical or biological molecules of the frontier electronic orbitals - the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO). These orbitals are the ones most active in chemical reactions, and are conceptually very similar to the valence band and conduction band states of semiconductors. Substitutional impurities will often introduce new “deep” electronic states into the gap between the HOMO and LUMO states of the original molecule and these states will have the following properties. (i) The likelihood that an impurity will produce a state in the gap depends on the site of the impurity. (ii) For impurities on a particular site, the wavefunction of the gap state will be relatively independent of the impurity. (iii) Only a small fraction of the deep level wavefunction (typically 10%) will lie within the impurity's cell and the largest part (about 40-50%) will be on the impurity's nearest-neighbor atoms. (iv) The derivative of the energy of the impurity state with respect to the central-cell impurity potential dE/dV is approximately equal to the probability of the gap-state electron being found on the impurity's site.
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  • 34
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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: To summarize the significance of the Sanibel Symposia in the field of “Structure and Activity of Biomolecules” is a Herculean task, meaning summarizing 30 years of the development of Quantum Biology in 30 minutes, which gives 1 minute per year. How does one do it? There are obviously many ways in which this cannot be done. One a priori conceivable way was to determine the most important (in the lecturer's eyes) contribution, discovery or paper for each of these consecutive 30 years and to present a list of potential laureates of what we could consider as an imaginary yearly Löwdin Prize. The author has tried the game but found it impraticable. A more reasonable solution seemed to be to trace the history of the main intellectual lines in the methods and themes which prevailed in the development of our knowledge on the Structure and Activity of Biomolecules during the period involved. It quickly turned out, however, that the scope was again so wide and its snapshot representation, the only feasible one, so dense and compact that it would have been reduced to the mere enumeration of the dozens of important, fundamental techniques and problems which have been dealt with during that time. So the author gave up this project too. Finally, after much hesitation, his choice went to a third procedure: the selection of the most important intellectual concept which had the most important impact on the development of the quantum-mechanical contribution to our understanding of structure-activity relationship in biomolecules. I believe this concept to be that of the electrostatic molecular potential, the introduction of which enabled the decisive step of moving over from the study of the structure and activity of small biomolecules or small constituents of the fundamental biomacromolecules to the study of these properties in the biomacromolecules themselves. Examples will be given which illustate this opinion.
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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: Molecular dynamics calculations have been used to probe the structure of a small sodium dodecylsulfate micelle in aqueous solution. The simulation system consisted of 42 dodecylsulfate ions, 42 sodium ions, and 1901 deuterated water molecules in a periodically replicated box. Electrostatic interactions were treated with an Ewald method. Over the course of a 182 ps trajectory, the micelle remained compact and roughly spherical with the hydrocarbon chains on the inside and the sulfate groups on the outer perimeter. The micelle displayed only small fluctuations in shape and had an average radius of about 15 Å. Relatively little water penetrated beyond the head-group region and the inner core (9 Å in radius) was devoid of water. On average each chain was found to have about three gauche defects. The average chain conformation in the micelle interior has been contrasted with that of an isolated monomer in solution. The counter ion distribution has been examined and compared with results from a previous simulation of a sodium octanoate micelle.
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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: Molecular mechanics calculations have been used to study the external noncovalent complexes formed between DNA and the strong carcinogen, aflatoxin B1. Three different sequences of hexameric duplexes were used for the DNA. Both the aflatoxin B1 parent molecule and its ultimate carcinogenic form, a carbocation, were modeled. The results are compared with recent experimental data on sequence specificity of the covalent attachment of aflatoxin to sites on the nucleotide bases. The comparison is discussed in light of a recently proposed hypothesis suggesting that the locus of carbocation formation is in acidic domains near the surface of the macroion and this determines the site of covalent adduct formation.
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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
    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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  • 38
    ISSN: 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 hexapeptide, (APGVGV), is one of the repeating sequences found in the natural protein, elastin. The cyclododecapeptide, cyclo(L-Ala1-L-Pro2-Gly3-L-Val4-Gly5-L-Val6)2, was synthesized and was found to undergo an inverse temperature transition leading to crystallization. Extensive NMR experiments have been carried out previously to evaluate its secondary structure in comparison to that of its linear counterpart, polyhexapeptide (APGVGV)n, directly and in terms of the concept of cyclic conformations with linear conformational correlates. Building on the previous work, this report adds 2-D NMR and molecular dynamics studies on the molecular structure of the cyclododecapeptide. From the 2-D COSY and 2-D NOESY experimental results, a reversal of the previous Gly3 and Gly5 resonance assignments in the cyclododecapeptide is reported. And in the 2-D NOESY study, a prominent NOE is found between the NH's of the two valine residues at position 4 and 6. Because the Val4 and Val6 residues are not adjacent to each other sequentially, the proximity of these two backbone hydrogens is used as an imposed constraint in constructing the molecular structure of the cyclododecapeptide. Molecular dynamics investigations were employed using the information from the 1-D and 2-D NMR experiments to arrive at potential molecular structures of the cyclododecapeptide. The characteristic secondary structural feature obtained is the expected Type II Pro2-Gly3 β-turn consisting of the 10-membered hydrogen bond between the residue-4 NH and the residue-1 C=O, a feature previously found in this and all other repeating peptides in elastin which contain the Pro2-Gly3 sequence.
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  • 39
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    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 40
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 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: 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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  • 42
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Preliminary results from our charge self-consistent LCAO band structure (CSCBS) calculations with Bloch sums as the basis reveal that a noncorrosive reduced band gap electrode for photoelectrochemical solar cells may be produced from a (1:1) mixture of β-PbO2 and TiO2 (both rutile). The band gaps for the constituents (β-PbO2 and TiO2) and the 1:1 mixture are calculated and a detailed characterization of the valence and the conduction bands is undertaken to offer a possible mechanism for the reduction of the band gap of the mixture. The band gap for the perovskite PbTiO3 is also calculated to offer a guideline for selecting from the competing pathways to the fabrication of noncorrosive photoelectrochemical electrodes.
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  • 43
    ISSN: 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 the study of three specific molecules, Na3(C2E″), HCN(X1Σ+), and O3(D1B2), each showing a distinctly different dynamical behavior at the energies studied, we show how techniques of nonlinear classical mechanics can be applied to simulate low resolution vibrational spectra and to extract the underlying dynamics from them. Particularly the importance of the single unstable periodic orbits in the chaotic regions of phase space is demonstrated.
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  • 44
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The two-dimensional (2D) electron gas present in Si δ-doped layers in p-type GaAs is investigated through photoreflectance (PR) measurements performed at 300 and 77 K. The obtained spectra show oscillatory structures above the GaAs band gap which are ascribed to Franz-Keldysh oscillations. The decrease in the energy differences of the oscillations extrema with temperature indicates a reduction of the built-in electric field in the δ-doped region. The values of the electric field extracted from the experimental data are compared with those obtained from theoretical calculations based on a simultaneous self-consistent solution of the Schrödinger and Poisson equations. The agreement between theory and experiment strongly indicate that the observed oscillatory structures in the PR spectra are due to transitions to electron states of the 2D electron gas.
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  • 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: A general discussion is provided on the changes in shape of a continuum of molecular surfaces along a reaction path. The path is partitioned in terms of the shape characteristics of the nuclear configurations. The most important feature of this partitioning is the eventual occurrence of a region where more than one shape can be associated with the molecular surface continuum. This region is a formal boundary range between different molecular shapes. Position and width of this range, as well as other related parameters, are introduced as measures of structural similarity along a reaction path. Examples of simple triatomic reactions are considered in order to show the relation between these similarity descriptors and the change of energy content along the path.
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  • 46
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    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 47
    ISSN: 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 are presented from linear combination of Gaussian type orbitals-model core potential-local spin density calculations on equilibrium geometries and harmonic frequencies of several small main group and organometallic molecules. The equilibrium geometries were obtained by minimizing the norm of an approximate analytical energy gradient while the frequencies were obtained by numerical differentiation of these gradients. Comparison with experimental data indicates that the approximate gradients yield accurate results which typically agree better with experiment than those of the Hartree-Fock method and compare favorably with commonly used correlated techniques.
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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: 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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  • 49
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 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: The matrix element for two-photon absorption is calculated as a residue of the general quadratic response function for a system subjected to an external time-dependent field. The matrix element is correct through first-order in electron correlation, and includes contributions in addition to those considered in previous calculations which were based on a truncated form of the Random Phase Approximation (RPA). Equations of motion are derived from the spectral representation of quadratic response, leading to equivalent expressions for two-photon absorption in the dipole length, mixed, and dipole velocity formulations. Because the transition moments evaluated in the three formalisms must be equivalent in a complete basis, the calculated results provide a measure of the adequacy of the basis set representation.As a test case, the X1σg+ (v = 0) → (E, F)1Σg+ (v = 6) two-photon process of H2 is considered. It is first demonstrated that agreement between length and velocity forms of the two-photon transition moment is obtained only through the consideration of all contributing terms. The vibrationally-averaged transition moment is decomposed into individual state contributions. It is found that the B1Σu+ and C1Πu-coupled matrix elements are comparable at the RPA level to values obtained at the near-full CI limit, and, although there is some disparity as to the relative weight of the remaining states, the total transition moment in the two formalisms are in good agreement. Using the transition moment obtained at the excitation frequency 1/2ωf, the integrated two-photon excitation cross-section is calculated. The value is in excellent agreement with the CI result and lies near the experimental error bars of the absorption cross-section determined from a single-beam measurement.
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  • 51
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dipole oscillator strength distributions (DOSDS), which are reliable in the sense of yielding accurate results for the various dipole properties of molecules, can be constructed if sufficient input information is available. The DOSDS are constructed from extensive experimental and theoretical information, including oscillator strength and photoabsorption cross section data, and are constrained to reproduce accurate refractivity measurements of the relevant dilute gases and to satisfy oscillator strength sum rules. The techniques for obtaining the DOSDS, and related properties, are well established for the evaluation of reliable isotropic dipolar dispersion energies. In this paper they are extended to the evaluation of anisotropic dipole properties, with particular emphasis on anisotropic dispersion energies. The approach used will be illustrated with applications involving H2, N2, and the rare gases. Also included will be brief discussions on (i) the use of reliable results for the dipole-dipole and triple-dipole dispersion energies in constructing complete two- and many-body potential energies and (ii) the advantages of constrained DOSD methods, relative to other approaches, for determining all the dipole properties of molecules.
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  • 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: We review the nature of the problem in the framework of Rayleigh-Schrödinger perturbation theory (the polarization approximation) considering explicitly two examples: the interaction of two hydrogen atoms and the interaction of Li with H. We show, in agreement with the work of Claverie and of Morgan and Simon, that the LiH problem is dramatically different from the H2 problem. In particular, the physical states of LiH are higher in energy than an infinite number of discrete, unphysical states and they are buried in a continuum of unbound, unphysical states, which starts well below the lowest physical state. Claverie has shown that the perturbation expansion, under these circumstances, is likely to converge to an unphysical state of lower energy than the physical ground state, if it converges at all. We review, also, the application of two classes of exchange perturbation theory to LiH and larger systems. We show that the spectra of three Eisenschitz-London (EL) class, exchange perturbation theories have no continuum of unphysical states overlaying the physical states and no discrete, unphysical states below the lowest physical state. In contrast, the spectra of two Hirschfelder-Silbey class theories differ hardly at all from that found with the polarization approximation. Not one of the EL class of perturbation theories, however, eliminates all of the discrete unphysical states. The best one establishes a one-to-one correspondence between the lowest energy states of the unperturbed and perturbed Hamiltonians, and a one-to-two correspondence for the higher states. We suggest that the EL class perturbation theories would be good starting points for the development of more effective perturbation theories for intermolecular interactions.
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  • 53
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present results of selfconsistent, first-principles calculations of total energies for AsGa and SbGa in GaAs. We confirm that both impurities in the substitutional Td site behave as double donors, and the first internal excitation appears at around 1 eV. For the neutral systems we obtain a metastable minimum in the total energy surface in a configuration with the impurity atom displaced toward the interstitial site; the transformation to this metastable configuration, however, is not expected to be operative for the SbGa defect.
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  • 54
    ISSN: 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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  • 55
    ISSN: 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 propagation of nonlinear waves induced by a shock impulse in a linear atomic chain of finite but large length is investigated numerically with a model interatomic potential. The shock impulse is initiated by giving the first atom in the chain an initial velocity vi toward its neighbor. We find that there exists a velocity vc such that for vi 〉 vc a soliton is produced by this impulse with a constant energy and supersonic velocity vs. Studies of the motion of the atom during the passage of this soliton reveal a behavior similar to that expected from the collision of hard spheres. However, for vi 〈 vc the induced pulse propagates below the speed of sound and gradually disperses through the emission of phonons. In the supersonic regime, vi 〉 vc, we find that there exists a velocity vm such that if vi 〉 vm, then vs 〈 vi, but if vi 〈 vm, then vs 〉 vi. Finally, in agreement with earlier studies using other model potentials, we find that if vi is large enough, a soliton can be generated with sufficient energy to spall an atom from the end of the chain.
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  • 56
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    New York, NY : Wiley-Blackwell
    ISSN: 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: Electron propagator calculations of Sc and Ti ionization energies employing contractions based on atomic natural orbitals reveal the soundness of this approach for choosing basis sets. Errors in total energies greatly exceed those for ionization energies, where contraction errors are reduced to less than 1 mhartree. Systematic improvements involve retention of natural orbitals with lower occupation numbers and augmentation with diffuse and polarization functions.
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  • 57
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 58
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 computations were carried out to determine the tunneling rates in the case of coupled motion of two protons along the reaction coordinate. The following molecular systems were studied for medium intermolecular distances (A - B = 2.72 or 2.75 Å); +AH - BH - A, where A was NH3 or H2O and BH was HF or H2O. In the cases where the bridge was HF, solvation was modeled with just one water molecule attached to each side of the perpendicular axis through HF at 2.75 Å. Coupled motion of three protons was also included in the case of H3O - H2O - H2O - H2O.
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  • 59
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    New York, NY : Wiley-Blackwell
    ISSN: 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.
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    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 conformation of numerous chemical compounds is strongly influenced by solvents. Knowledge of their structure in solution is necessary, especially for a discussion of the biological and pharmacological activity of the molecules. The neurotransmitters and their agonists and antagonists are known to be flexible molecules that interact with quite distinct receptors. The conformational properties of several GABA (γ-aminobutyric acid) mediators have been studied by the MNDO technique. The optimized geometries of the molecules have been used to study the solvent effects on their conformational properties considering the supermolecule approach for their first hydration shell. A conjectural pharmacophoric pattern for several GABA inhibitors has already been suggested from the molecular electrostatic potentials (MEP) of several molecules using a localized bond orbital technique. In the present work, MEP calculations have been carried out considering a solvent effect on the MNDO optimized geometries to investigate any deviation from the earlier results.
    Additional Material: 8 Ill.
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  • 61
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 62
    ISSN: 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 generating function approach based on molecular orbital graph theory is presented that provides a straightforward way of obtaining the secular polynomials and energy bands for repeated unit systems from polynomial recurrence expressions. The possibility of obtaining the analytical energy-level spectrum of the system can also be predicted. These results are then used to discuss the vibrational problems of finite chain systems with single- and double-component lattices. It seems to be the first report describing the vibrational states of an (AB)N chain.
    Additional Material: 1 Ill.
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  • 63
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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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  • 64
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 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 a way of partitioning of Mulliken overlap populations, the hybrid orbitals of central atoms and ligands are obtained by the CNDO method. The dipole moments of some hybrids are calculated using the hybrid orbitals obtained, and they are mostly in accordance with experimental values. In addition, overlap integrals may be decomposed into components, and then σ, π, and δ hybrid orbitals can be obtained.
    Additional Material: 2 Ill.
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  • 65
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: To model a real 20-component protein chain, the results of the electronic density of states (DOS) of seven-component periodic and aperiodic polypeptide chains calculated on the basis of ab initio matrix block negative factor counting method are presented. The seven amino acid residues taken into account are rather different: glycine, serine, cysteine, asparagine, histidine, aspartic acid, and tryptophane. In the case of aperiodic chains, the energy states are found to be distributed over a much wider range of energy than in the corresponding periodic chains. In the intrinsic state, both periodic and aperiodic polypeptide chains are predicted to be insulators. The effect of the block copolymerization on the DOS distribution and the band gap values of both periodic and aperiodic polypeptide chains has been also investigated. Finally, the methods to calculate from the DOS curves hopping conductivities and the possibilities to compare them with experiment are outlined.
    Additional Material: 8 Ill.
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  • 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
    Type of Medium: Electronic Resource
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  • 67
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 68
    ISSN: 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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