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  • 1
    Electronic Resource
    Electronic Resource
    Anisotropic rigid rotor potential energy function for H2O–H2 (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5824-5830 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have calculated the interaction for H2O–H2 at 722 points on a five-dimensional surface where both molecules are treated as rigid rotators and we have fitted the ab initio points to a 48-term angular expansion of products of spherical harmonics and rotation matrices. The resulting potential energy function shows strong angle dependence with a large contribution from electrostatic interactions. When averaged over H2 orientations, the resulting water-atom-like surface is found to have zero crossing and minimum at similar distances to the corresponding H2O–He surface but to be generally more repulsive at short range and more attractive at long range. The isotropic average of the potential has a zero-crossing radius σ=3.05 A(ring) and a well depth ε=49.5 cm−1 at an intermolecular separation Rm=3.52 A(ring).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467297
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  • 2
    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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  • 3
    Electronic Resource
    Electronic Resource
    The singlet–triplet energy separation in silylene (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5117-5119 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: MCSCF calculations followed by large scale configuration interaction, comparable with the best done for isovalent methylene, give a singlet–triplet splitting in silylene of 21.0±1 kcal/mol. The singlet is the lower energy state. Structural parameters for the as yet unobserved triplet are re =1.48±0.005 A(ring) and θe =118.5±1°.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451704
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  • 4
    Electronic Resource
    Electronic Resource
    Is N-protonated hydrogen isocyanide, H2NC+, an observable interstellar species? (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 5194-5199 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio molecular orbital theory is used to examine the singlet and triplet potential energy surfaces for the CH2N+ system. The results confirm those of earlier studies which suggested that the singlet H2NC+ isomer could be formed via the corresponding triplet isomer. Also, it is shown that the reaction HCN++H2 might lead to this metastable isomer without invoking the triplet species. The best test of the hypothesis that this molecule can be formed by gas phase, ion molecule reactions and may be an important precursor in the interstellar synthesis of HCN and HNC is to search for it in space. To this end, theoretical predictions are made of its rotational frequencies and its vibrational frequencies and intensities to serve as a guide to laboratory spectroscopists and radioastronomers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451683
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular orbital predictions of the vibrational frequencies of some molecular ions (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 333-341 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent spectroscopic advances have led to the first determinations of infrared vibration-rotation bands of polyatomic molecular ions. These initial detections were guided by ab initio predictions of the vibrational frequencies. The calculations reported here predict the vibrational frequencies of additional ions which are candidates for laboratory analysis. Vibrational frequencies of neutral molecules computed at three levels of theory, HF/3-21G, HF/6-31G*, and MP2/6-31G*, were compared with experiment and the effect of scaling was investigated to determine how accurately vibrational frequencies could be predicted. For 92% of the frequencies examined, uniformly scaled HF/6-31G* vibrational frequencies were within 100 cm−1 of experiment with a mean absolute error of 49 cm−1. This relatively simple theory thus seems suitable for predicting vibrational frequencies to guide laboratory spectroscopic searches for ions in the infrared. Hence, the frequencies of 30 molecular ions, many with astrochemical significance, were computed. They are CH+2, CH+3, CH+5, NH+2, NH+4, H3O+, H2F+, SiH+2, PH+4, H3S+, H2Cl+, C2H+, classical C2H+3, nonclassical C2H+3, nonclassical C2H+5, HCNH+, H2CNH+2, H3CNH+3, HCO+, HOC+, H2CO+, H2COH+, H3COH+2, H3CFH+, HN+2, HO+2, C3H+, HOCO+, HCS+, and HSiO+.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448805
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  • 6
    Electronic Resource
    Electronic Resource
    Calculations on the competition between association and reaction for C3H++H2 (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 2812-2820 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A potential energy surface has been calculated for the competing associative and reactive ion–molecule processes involving the reactants C3H++H2. Our ab initio results show that the linear ion C3H+ and H2 can directly access the deep potential well of the propargyl ion H2CCCH+, which is calculated to lie 390 kJ mol−1 below the zero-point energy of the reactants. Isomerization between the propargyl ion and the lower energy, cyclic C3H3+ ion, calculated to lie 501 kJ mol−1 below the zero-point energy of reactants, can subsequently occur via two pathways. One of these pathways involves a transition state lying 22 kJ mol−1 below the energy of the reactants while the other, which occurs at much lower energies, involves two transition states and an intermediate. The dissociation of c-C3H3+ into c-C3H2++H is calculated to occur directly, without any intermediate potential energy maximum, but the energy of the products lies 7.3 kJ mol−1 above the energy of the reactants. Using the minimum energy potential pathway and properties of the stationary point structures determined via ab initio methods, we have calculated both the association rate coefficient to produce C3H3+ as a function of density and the branching ratio between the propargyl and cyclic structures of the ion. Our results are in good agreement with some experimental results and in conflict with others. Specifically, we agree with the 1:1 branching ratio measured for the propargyl and cyclic isomers of C3H3+ at 80 and 300 K and we agree with the rate coefficient for radiative association measured at 80 K. We cannot reproduce reported measurements that the reactive channel (C3H2++H) is the dominant channel at 80 K and at low gas densities, or that the association channel at high densities saturates at an effective rate coefficient well below the Langevin value −2×10−11 cm3 s−1 at 300 K and 1×10−10 cm3 s−1 at 80 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465190
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  • 7
    Electronic Resource
    Electronic Resource
    Comment on broadening of water microwave lines by collisions with helium atoms (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 8150-8156 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Earlier theoretical descriptions of interaction forces and collision dynamics in the H2O–He system [A. Palma et al., J. Chem. Phys. 89, 1401 (1988); S. Green et al., ibid. 94, 1346 (1991)] are improved by calculating the interaction at additional orientations and by including diffuse functions in the molecular orbital expansion basis set. The improvements make substantial changes to the potential energy surface, but make only small changes in predicted microwave pressure broadening coefficients. The qualitative temperature dependence and relative widths of different lines appear to be converged with respect to successive improvements to the theoretical description of this system. Remaining discrepancies with available data must reflect problems with the experimental values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462318
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  • 8
    Electronic Resource
    Electronic Resource
    Calculations of H2O microwave line broadening in collisions with He atoms: Sensitivity to potential energy surfaces (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1346-1359 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Broadening parameters for three microwave lines of water at 22.2, 183.3, and 380.2 GHz, in a bath of helium atoms, are calculated using accurate molecular scattering S matrices obtained from two theoretical potentials presented by Palma et al., J. Chem. Phys. 89, 1401 (1988). For the 22 GHz line results are in substantial agreement with values presented in that work, indicating the accuracy of approximate methods used there. The present work improves the potential energy surfaces, computed from perturbation theory (MP4) and variational interacting correlated fragments (ICF1) wave funcitons, by correcting them for basis set superposition error (BSSE), and recomputes the line broadening using a different procedure for fitting computed energy points. In addition, the entire set of calculations are repeated with a quite different basis set for orbital expansion to establish the reliability of the potential energy surface. We show that adjustments for superposition error are essential, and that broadening cross sections computed from the new surfaces are changed 10%–30% from the old, significantly improving agreement with experiment. The MP4 BSSE adjusted surface appears to be the most accurate, giving room temperature broadenings of 8.9, 11.8, and 10.0 A(ring)2 compared with experimental determinations of 12.2±1.2 , 11.9, and 11.2 A(ring)2 for the 22, 183, and 380 GHz lines, respectively. Thus, computed line to line variation is larger than observed. The ICF1 BSSE adjusted results for pressure broadening cross section parallel those from the MP4 BSSE calculations but are about 10% smaller. We believe our computed results are stable with respect to basis set for orbital expansion and that the scattering calculations are accurate. Any theoretical inadequacy has been pinpointed to too few points on the potential energy surface resulting in an inadequate description of the angle dependence. It is not clear whether the present discrepancy between computation and experiment stems from this or from errors in the experimental values, although we show some indication that additional information on the surface might decrease the computed broadenings, worsening agreement with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459992
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  • 9
    Electronic Resource
    Electronic Resource
    On the dissociation energy of Mg2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 5377-5383 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The bonding in the X 1Σ+g state of Mg2 is investigated using near-complete valence one-particle Slater and Gaussian basis sets containing up to h functions. Full configuration interaction (FCI) calculations are used to calibrate four-electron correlation treatments. We show that the four-electron complete CI limit can be approached using a sequence of either second-order CI (SOCI) or interacting correlated fragment (ICF) calculations. At the valence level, our best estimate of the dissociation energy (De) is 464 cm−1. We show that this is a lower limit and probably within 5 cm−1 of the complete basis value. The inclusion of core–valence correlation using a model operator approach decreases De by about 35 cm−1 and increases the bond length by 0.03 a0, thereby yielding spectroscopic constants in good agreement with experiment. Attempts to compute the core–valence effect accurately by expanding the CI treatment were unsuccessful.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458515
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  • 10
    Electronic Resource
    Electronic Resource
    Ab initio determination of mode coupling in HSSH: The torsional splitting in the first excited S–S stretching state (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 5905-5909 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A mechanism for the enhanced splitting detected in the millimeter-wave rotational spectra of the first excited S–S stretching state of HSSH (disulfane) has been studied. The mechanism, which involves a potential coupling between the first excited S–S stretching state and excited torsional states, has been investigated in part by the use of ab initio theory. Based on an ab initio potential surface, coupling matrix elements have been calculated, and the amount of splitting has then been estimated by second-order perturbation theory. The result, while not in quantitative agreement with the measured splitting, lends plausibility to the assumed mechanism.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457459
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