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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11166-11175 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper presents a comparison of surface hopping and mean field approaches for simulating proton transfer reactions. In these mixed quantum/classical simulations, the transferring proton(s) are treated quantum mechanically, while the remaining nuclei are treated classically. The surface hopping method used for these calculations is the molecular dynamics with quantum transitions (MDQT) method based on Tully's fewest switches algorithm. In addition, this paper describes a modified MDQT method (denoted MDQT*) that eliminates classically forbidden transitions to promote consistency between the quantum probabilities and the fraction of trajectories in each adiabatic state. The MDQT, MDQT*, mean field, and fully quantum dynamical methods are applied to one-dimensional model single and double proton transfer reactions. Both the MDQT and MDQT* calculations agree remarkably well with the fully quantum dynamical calculations, while the mean field calculations exhibit qualitatively incorrect behavior. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8442-8454 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general minimal model for proton-coupled electron transfer (PCET) reactions in solution is presented. This model consists of three coupled degrees of freedom that represent an electron, a proton, and a solvent coordinate. Altering the parameters in this model generates a wide range of PCET dynamics. This paper focuses on three model systems corresponding to three different mechanisms: a concerted mechanism in which the proton and electron are transferred simultaneously, a sequential mechanism in which the proton is transferred prior to the electron, and a sequential mechanism in which the electron is transferred prior to the proton. The surface hopping method ‘molecular dynamics with quantum transitions' (MDQT) is applied to these model systems. The proton and electron coordinates are treated quantum mechanically, and the solvent coordinate is treated classically. Thus the adiabatic quantum states are two-dimensional wavefunctions that depend on both the electron and the proton coordinates. The MDQT method incorporates nonadiabatic transitions between these mixed proton/electron adiabatic quantum states. The MDQT simulations presented in this paper provide insight into the fundamental physical principles and the dynamical aspects of PCET reactions. Nonadiabatic effects are shown to play an important role in determining the rates and mechanisms of PCET reactions. This represents the first application of MDQT to a system in which both a proton and an electron are treated quantum mechanically. © 1997 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 1944-1954 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational predissociation of a van der Waals complex (Cl2Ne) is studied using a method based on the multiconfiguration time-dependent Hartree approximation. The three-dimensional wave function is first expanded to the time-independent Cl2 vibrational bases and the Hartree approximation is then imposed on the channel wave functions. The wave packets are propagated for a few picoseconds and five configurations are found to give convergent results. The decay lifetimes, product state distributions and the wave packet dynamics are compared with exact results and the agreement is found to be generally satisfactory. It is found that the decay depends sensitively on the quality of the initial resonance wave function and the single configuration TDH gives only a crude approximation of the dissociation dynamics. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 9072-9082 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two-dimensional time-dependent wave packet calculations are carried out on a collinear model of the I2(B)–Ar complex to investigate the possible kinematic origin of the one-atom cage effect in small van der Waals molecules. Three different excitation wavelengths are considered (496.5, 488, and 476.5 nm), and the dynamics are assumed to be restricted to the I2 B state electronic surface, with no nonadiabatic transitions following the pump excitation. Good agreement with experiment is obtained. To investigate the sensitivity of observable final state distributions on the weak intermolecular potential between I2 and Ar, three slightly different B state I–Ar interactions are employed for the case of 488 nm excitation. It is found that relatively small changes in the form and magnitude of the weak van der Waals interactions can have a large effect on the final state distributions. These results suggest that the experimental data on I2–Ar photodissociation–recombination can be explained by a purely kinematic one-atom cage effect on the B state electronic surface for a collinear population of I2–Ar clusters, without the need to introduce nonadiabatic electronic effects. © 1996 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7809-7821 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An empirical two-plus-three-body potential, developed by Murrell and co-workers [J. N. Murrell and R. A. Rodriguez-Ruiz, Mol. Phys. 71, 823 (1990)], is applied to the study of fcc aluminum. The parameters in the potential are derived by fitting the experimental phonon-dispersion curves and elastic constants. Calculations, using this potential, on a number of one-, two- and three-dimensional extended systems give results which are in quantitative agreement with recent ab initio calculations [I. J. Robertson, M. C. Payne, and V. Heine, Europhys. Lett. 15, 301 (1991)]. Calculations on small- and medium-sized aluminum clusters give cluster geometries and growth patterns which agree qualitatively with previous ab initio molecular-orbital and density-functional studies.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5831-5840 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A multiconfiguration time-dependent Hartree method is applied to study the photodissociation dynamics of methyl iodide on a MgO surface. The surface is assumed to be rigid and the dissociation is restricted in orientations parallel to the surface normal. Two active mathematical dimensions and two electronic states are included in our model. The dynamics of the fragments (the methyl radical and iodine atom) are complicated by the so-called "chattering effect'' and by nonadiabatic transitions between the two dissociative states of CH3I. It is shown from the comparison with exact calculations that the single configuration time-dependent Hartree approximation fails to give an accurate description of the dynamics. The failure of the Hartree approximation, which is due to the neglect of direct spatial correlations between modes, can be effectively corrected by including a few additional configurations. Our results show that four configurations are sufficient to provide an accurate description of the complex dissociation dynamics of the CH3I/MgO system.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1231-1241 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation dynamics of HBr adsorbed on a LiF(001) surface are investigated using both exact and time-dependent Hartree (TDH) methods on realistic potential energy surfaces. The dissociation dynamics are restricted in two dimensions and two coupled dissociative states of the adsorbate are included. The wave packets are propagated on numerical grids, and fast Fourier transform (FFT) and discrete variable representation (DVR) are used to calculate the action of the Hamiltonian. In the TDH treatment, each excited electronic state is represented by a single nuclear configuration. Final radial, angular, and momentum distributions of the H fragment are calculated. Comparisons between the exact and TDH results reveal that the agreement between the two is generally reasonable and is better for highly averaged quantities. Results also show that nonadiabatic transition dynamics are correctly reproduced by the TDH approximation. Finally, the calculated results are found consistent with the experimental observations.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 7051-7063 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Numerical tests are presented for a method that combines the time-dependent self-consistent-field (TDSCF) method with the reaction path Hamiltonian (RPH) derived by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)]. The theoretical basis for this TDSCF-RPH method was presented in a previous paper. The equations of motion were derived for three different cases: (1) zero coupling matrix (i.e., zero reaction path curvature and zero coupling between the normal modes); (2) zero reaction path curvature and nonzero coupling between the normal modes; and (3) zero coupling between the normal modes and nonzero but small reaction path curvature. For these three cases the dynamics can always be reduced to a one-dimensional numerical time propagation of the reaction coordinate. In this paper the TDSCF-RPH methodology for all three cases is tested by comparing the TDSCF-RPH dynamics to exact quantum dynamics based on the exact Hamiltonian for simple model systems. The remarkable agreement indicates that the TDSCF-RPH method could be useful for the calculation of the real-time quantum dynamics of a wide range of chemical reactions involving polyatomic molecules. © 1998 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 7085-7099 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method that combines the time-dependent self-consistent-field (TDSCF) method with the reaction path Hamiltonian (RPH) derived by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)] is proposed. This TDSCF-RPH method allows the calculation of the real-time quantum dynamics of chemical reactions involving polyatomic molecules. When both the coupling between the normal modes and the curvature are zero, the dynamics of an F-dimensional system is shown to reduce to a one-dimensional numerical time propagation. When the reaction path curvature is zero and the coupling between the normal modes is non-zero, the dynamics is shown to still reduce to a one-dimensional problem for a specific choice of initial wavepacket (which can have an arbitrary component for the reaction coordinate), but F coupled one-dimensional equations of motion must be propagated for a general initial wavepacket (unless the RPH is transformed to the diabatic representation). When the coupling between the normal modes is zero and the reaction path curvature is non-zero but small, the dynamics is shown to reduce to a one-dimensional numerical time propagation for an arbitrary initial wavepacket. The derivations of the equations of motion for these cases are presented in this paper, and numerical tests are presented in a separate paper. © 1998 American Institute of Physics.
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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The extension of the surface hopping method "molecular dynamics with quantum transitions" (MDQT) to double proton transfer and proton-coupled electron transfer reactions is tested by comparison to fully quantum dynamical calculations for simple model systems. These model systems each include four potential energy surfaces and three or four avoided curve crossings. The agreement between the MDQT and fully quantum dynamical calculations provides validation for the application of MDQT to these biologically important processes. © 1997 American Institute of Physics.
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