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  • 1
    Electronic Resource
    Electronic Resource
    Perturbative approaches to highly excited molecular vibrations of H2O, D2O, and HDO (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1893-1901 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular vibrations of water are studied using Van Vleck perturbation theory. In these calculations, the OH stretches are expressed in terms of the Morse coordinate, yi =[1−exp(−αri)]/α, and its conjugate momentum, while the bend is treated in a traditional manner. Nearly degenerate perturbative calculations are presented for bond and normal-mode representations. The eigenvalues agree well with those obtained variationally. The coefficients of the perturbatively derived, effective Hamiltonians are compared to those of spectroscopic Hamiltonians with similar structural forms. The former are derived from the potential of Halonen and Carrington [J. Chem. Phys. 88, 4171 (1988)], whereas the latter are treated as spectral fitting parameters. The fit parameters are accurately reproduced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458071
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  • 2
    Electronic Resource
    Electronic Resource
    Calculation of infrared intensities of highly excited vibrational states of HCN using Van Vleck perturbation theory (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3488-3493 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical Van Vleck perturbation theory (CVPT) is used to calculate electric dipole intensities for one-, two-, and three-dimensional models of HCN and a six-dimensional model of H2CO. Lehmann and Smith [J. Chem. Phys. 93, 6140 (1990)] have shown that the intensities of overtone transitions are sensitive to the details of the inner wall of the potential. Dipole intensities calculated for several, similar one-dimensional CH stretch potentials demonstrate that perturbation theory correctly predicts this sensitivity. The perturbation intensities of a two-dimensional ab initio dipole surface indicate the importance of selected stretch–stretch resonance interactions in interpreting the CH stretch overtone spectra of HCN. The inclusion of the bend confirms that this degree of freedom plays a significant role in weakening the intensity of the CN overtones. The CH stretch overtone spectra of H2CO is calculated to illustrate the utility of the perturbative approach for predicting the transition intensities for a system in which there are multiple Fermi interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460851
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  • 3
    Electronic Resource
    Electronic Resource
    Rotation–vibration interactions in highly excited states of SO2 and H2CO (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7449-7465 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical Van Vleck perturbation theory (CVPT) is used to investigate rotation–vibration mixing of highly excited vibrational states of SO2 and H2CO. For SO2 we find a nearly complete separation of the rotational and vibrational degrees of freedom, even for J=12 and Evib=11 000 cm−1. In contrast, for H2CO we observe extensive mixing between rotational and vibrational degrees of freedom at similar rotational excitation but with Evib=8000 cm−1. Although a-axis Coriolis coupling is pronounced, b- and c-axis Coriolis couplings play an important additional role in mixing states with different Ka quantum numbers. The implementation of CVPT, the choice of internal coordinates, and the convergence of the results are discussed in detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461371
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  • 4
    Electronic Resource
    Electronic Resource
    Determining potential-energy surfaces from spectra: An iterative approach (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 2938-2947 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general method for iteratively fitting the coefficients of a Taylor-series expansion of the potential-energy surface for a polyatomic molecule to the observed transition frequencies and rotational constants is presented. This approach utilizes the efficiency of fourth-order Van Vleck perturbation theory for calculating these properties, scaled to the results of converged variational calculations. Three fits to the transition frequencies and rotational constants obtained from absorption and stimulated emission pumping spectra of HCN are presented. Comparison of these potentials sheds light on the uniqueness of fit potential surfaces, given this set of observables. We fit the vibrational energies for 71 states with a mean absolute deviation of 0.69 cm−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463035
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  • 5
    Electronic Resource
    Electronic Resource
    Perturbative calculations of vibrational (J=0) energy levels of linear molecules in normal coordinate representations (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3476-3487 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Canonical Van Vleck perturbation theory is used to transform curvilinear and rectilinear normal coordinate vibrational Hamiltonians of HCN, C2H2, and CO2 to block-diagonal effective Hamiltonians. Accurate energies as high as 11 000 cm−1 above the zero point are reported for all three molecules. In the absence of off-diagonal coupling terms in the effective Hamiltonians, these two coordinate systems yield identical perturbative expansions for the vibrational energies. Only when coupling terms are introduced do differences between the calculated energies in the two representations become apparent. In CO2, where there is pronounced configuration interaction between nearly degenerate states, we find that the perturbative energies obtained from the curvilinear normal coordinate Hamiltonian are converging significantly faster than those obtained in the rectilinear normal coordinate representation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460850
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  • 6
    Electronic Resource
    Electronic Resource
    Transition state dynamics of chemical reactions in clusters: A six-dimensional study of Ar(ClHCl) (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 986-998 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cluster effects on transition state dynamics are investigated through a time-dependent calculation of photodetachment spectrum of Ar(ClHCl)−. This system is studied by propagating a three-dimensional grid representation of the wave function in the H atom coordinates, coupled to three one-dimensional semiclassical Gaussians in the three lowest frequency normal coordinates of Ar(ClHCl)−. Over the 75 fs propagation times considered here, the motions of the three heavy atoms are very small. Therefore, we expect that the coupled quantum/semiclassical propagation will be in quantitative agreement with a full quantum treatment of this system. The couplings between the quantum and semiclassical degrees of freedom and between the three semiclassical degrees of freedom are introduced through the time-dependent self-consistent field (TDSCF) approximation. A computational bottleneck in applying the TDSCF approximation to such systems is that propagation of each of the modes requires the evaluation of the average of the potential over the wave functions in the three remaining modes. A solution to this problem for H transfer systems is presented and discussed. Comparisons between the dynamics of Ar(ClHCl) and ClHCl indicate that the presence of the Ar atom produces a hole in the wave function that is otherwise cylindrically symmetric. Possible observable experimental consequences of the introduction of the Ar atom to this system are discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469799
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  • 7
    Electronic Resource
    Electronic Resource
    An accurate quartic force field for formaldehyde (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 480-487 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor-series expansion of the potential-energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (≤7600 cm−1) for 138 states with an absolute mean deviation of 1.5 cm−1. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471531
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  • 8
    Electronic Resource
    Electronic Resource
    A quantitative approximation for the quantum dynamics of hydrogen transfer: Transition state dynamics and decay in ClHCl− (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1975-1987 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general method for studying transition state spectroscopy and dynamics in hydrogen atom transfer reactions is presented. This approach is based on the time-dependent self-consistent field (TDSCF) approximation and is applied to a study of the ClHCl− photodetachment experiments of Metz et al. [Metz et al., J. Chem. Phys. 88, 1463 (1988)]. Comparison of results of exact time-dependent and TDSCF calculations are made for collinear and three-dimensional (J=0) approximations for the quantum dynamics. When ClHCl is constrained to be collinear, the TDSCF calculation overcorrelates the motions in the H atom displacement and ClCl extension coordinates. This results in relatively poor agreement with the exact result for many properties of the wave function. In contrast, when the system is propagated in the three vibrational coordinates of the system, the transition state dynamics are effectively over much more rapidly. Consequently, the TDSCF approximation yields results of very good quantitative accuracy over the time required for most of the wave function to decay off of the transition state. Comparison is also made between the wave function that results from the exact propagation and from TDSCF when the wave function in the ClCl stretch coordinate is approximated by a Gaussian wave packet. Here the magnitude of the overlap between the two TDSCF wave functions in the H atom coordinates, for quantum and semiclassical propagations of the wave function in the ClCl distance coordinate, is greater than 0.98 over the time of the propagations. These TDSCF calculations are repeated for a wave function that is approximated by a product of a two-dimensional wave function in the hydrogen atom coordinates and a one-dimensional wave function in the ClCl extension coordinate and even better quantitative agreement with the exact propagation is achieved. The success of this method for studying ClHCl gives us confidence that TDSCF will provide a general powerful tool for studies of hydrogen and proton transfer reactions in large systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467707
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum Monte Carlo studies of small B(H2)n clusters (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2577-2591 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and stability of clusters of a boron atom with one to eight H2 molecules is investigated. For the simplest BH2 clusters, the lowest ab initio adiabatic potentials for o-H2 and p-H2 interacting with a boron atom are used. For the larger clusters (n=2–8), the p-H2 is treated as a sphere, and the total potential is taken to be the sum of pairwise additive B–H2 and H2–H2 interactions which include, in the former case, an anisotropy due to the orientation of the unpaired B 2p electron. This electronic interaction is considerably more attractive when H2 approaches the B atom in a plane perpendicular to the orientation of the 2p orbital. The local and global minima on these potential surfaces were located and diffusion quantum Monte Carlo simulations were used to determine the energies and properties of the ground state wave functions for these B–(H2)n clusters. For the B(H2) cluster, a comparison is made with the results of variational calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468432
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  • 10
    Electronic Resource
    Electronic Resource
    The bending dynamics of acetylene (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 459-468 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics and spectroscopy of (J=0) acetylene bending degrees of freedom are investigated using a reduced dimensional Hamiltonian. This Hamiltonian is obtained by applying an adiabatic approximation to average the vibrational Hamiltonian over the ground state in the three stretch coordinates. Within this approximation, an effective bend force field is obtained by adjusting force constants in the adiabatic potential to improve agreement between experimental and theoretical eigenvalues. With minor modification, a global bend force field is determined that qualitatively describes the vinylidene vibrations and quantitatively describes the acetylene vibrations. This surface is compared to the results of a recent ab initio calculation. A dispersed fluorescence spectrum out of the excited A˜ state, calculated from this model, is found to agree well with results of a recent experimental study. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471899
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