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  • 1
    Electronic Resource
    Electronic Resource
    Symmetry breaking in molecular calculations and the reliable prediction of equilibrium geometries. The formyloxyl radical as an example (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 3567-3576 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A systematic approach to symmetry breaking in molecular calculations, based on MCSCF and multireference CI (MRCI) wave functions, is presented. A series of MCSCF expansions is generated by successively incorporating resonance effects and size effects into the wave functions. The character of the potential surface obtained at each level is analyzed. As an example, the potential energy curves of the ground state (σ) and the first excited state (π) of the formyloxyl radical (HCO2) are characterized. The σ and π equilibrium structures are shown to be symmetric, with an adiabatic σ−π excitation energy of 9.2 kcal/mol. Unlike earlier theoretical studies, our MCSCF model produces a qualitatively correct potential surface. Therefore, we are able to extract reliable vibrational frequencies from the MRCI potential surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449162
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Interchannel coupling and ground state correlation effects in the photoionization of CO (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5097-5103 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a general procedure for applying the complex Kohn variational method to the calculation of molecular photoionization cross sections and asymmetry parameters. In this initial application of the method, we examine the effects of interchannel coupling and ground state correlation on the X 2Σ+(5σ−1), A 2Π(1π−1), and B 2Σ+(4σ−1) partial photoionization cross sections and asymmetry parameters for the CO molecule. We find that the dominant effect of interchannel coupling is to remove a spurious π→π* resonance feature from the continuum that appears at the frozen-core Hartree–Fock level. We also find that it appears to be important to combine the effects of final channel coupling with a correlated initial target state to achieve quantitatively correct cross sections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466010
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Ab initio multireference configuration interaction study of CH2NNO2. HONO elimination vs NN bond fragmentation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1857-1864 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Features of the potential energy surface pertinent to the thermal decomposition mechanism of methylenenitramine (CH2NNO2) are investigated. At issue are the activation energies both for concerted molecular dissociation leading to HCN and HONO and for the simple NN bond cleavage leading to CH2N and NO2. We use multiconfiguration (MC)SCF wave functions with double-zeta and double-zeta plus polarization quality basis sets to locate minimum energy and transition state structures as well as determine harmonic vibrational frequencies and resulting zero-point vibrational energies. The energetics are determined using configuration interaction wave functions which include single and double excitations from a selected set of reference configurations constructed with the MCSCF molecular orbitals. Our best calculations, without adjustment, predict the molecular dissociation to have an activation energy slightly lower than that for simple bond rupture. Calculations at the level of those reported here are known, however, to underestimate bond dissociation energies by several kcal/mol. Based on trends from past calculations, and new calculations on a related compound, we estimate the activation energy for the HONO elimination path to be 31±4 kcal/mol and the NN bond dissociation energy to be 35±4 kcal/mol. This reverses the energetic ordering of previous theoretical predictions and is consistent with recent molecular beam experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459063
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio study of low-energy electron-ethane scattering (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5480-5488 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the results of the first ab initio study of low-energy electron-ethane scattering which includes the effect of target polarization. The complex-Kohn method is used in scattering calculations that employ both static-exchange and polarized-self-consistent-field (polarized-SCF) trial functions. Integral, momentum transfer, and differential elastic cross sections are reported for both staggered and eclipsed conformations. Overall agreement between our studies and the most recent experimental data is very good. For staggered ethane, our integral cross section shows a Ramsauer–Townsend minimum at 0.18 eV. This is only the second theoretical study to find such a minimum in a molecule that possesses a nonzero quadrupole moment. We find that a polarized-SCF wave function is needed to obtain reliable cross sections in the vicinity of the Ramsauer–Townsend minimum and that this polarized-SCF wave function also provides a good description of the 7.5 eV f-wave shape resonance. The low-energy cross sections we obtain for staggered and eclipsed C2H6 differ very little from each other near the Ramsauer–Townsend minimum, but significant differences are found at higher energies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463781
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  • 5
    Electronic Resource
    Electronic Resource
    Triple intersection of H3 resonance states (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1756-1758 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In recent measurements of the cross-section for dissociative recombination of H+3 a strong enhancement feature was found which is several eV wide and centered near 9.5 eV incident electron energy. We show this peak is due to four autoionizing states of H3. We also find that three of these resonance states cross along the symmetric stretch seam of the potential energy hypersurface. Such triple intersections may be expected in any polyatomic system which is of high enough symmetry to allow degenerate molecular orbitals. Their existence can profoundly affect the nuclear dynamics on these surfaces in a manner similar to that found in the more common cases of conical intersections between two surfaces. The calculation and characterization of these states will be described.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466602
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  • 6
    Electronic Resource
    Electronic Resource
    A fixed-nuclei, ab initio treatment of low-energy electron-H2O scattering (1992)
    Springer
    The European physical journal 24 (1992), S. 117-124 
    Details
    ISSN: 1434-6079
    Keywords: 34.80.Gs
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report the results of a theoretical study of low-energy electron-H2O scattering. The calculations employ the complex Kohn variational technique and are undertaken at both the static-exchange and polarized-SCF levels. Target polarization effects are included by using an ab initio optical potential. Particular attention is paid to the complications attending electron scattering from target molecules that possess a permanent dipole moment. We describe the steps necessary to extract meaningful differential cross sections from fixed-nuclei calculations that ignore the rotational motion of the target. Comparison is made with experiment as well as other recent theoretical treatments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01426695
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    Paper (German National Licenses)
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