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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2404-2412 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work, we explore the applicability of a multiconfiguration version of the time-dependent Hartree approximation in photodissociation dynamics. A two-dimensional model system based on the A-band photodissociation of ICN is used. The propagation of the wave packet, which is expanded on grids, is carried out using pseudospectral methods and a predictor–corrector integrator. It is found that the quantum state resolved quantities, such as the final CN rotational distribution, require many more configurations than averaged quantities like the absorption spectrum. The calculated results from this work agree well with previous quantum calculations. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    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.
    Type of Medium: Electronic Resource
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  • 3
    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.
    Type of Medium: Electronic Resource
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  • 4
    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.
    Type of Medium: Electronic Resource
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  • 5
    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.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8933-8939 
    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.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5727-5739 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photoinduced proton-coupled electron transfer is investigated for a minimal model consisting of three coupled degrees of freedom that represent an electron, a proton, and a collective solvent coordinate. Altering the parameters in this model generates a wide range of proton-coupled electron transfer (PCET) dynamics. Four different models are presented in this paper. Three of these models represent sequential mechanisms and one represents a concerted mechanism. The adiabatic potential energy curves as a function of solvent coordinate and the corresponding two-dimensional wave functions, which depend on both the proton and the electron coordinates, are calculated in order to study the possible mechanisms of photoinduced PCET. The surface hopping method "molecular dynamics with quantum transitions" (MDQT), which incorporates nonadiabatic transitions between adiabatic quantum states, is utilized to simulate the dynamics of photoinitiated PCET for two of these model systems. In this application of MDQT the proton and electron coordinates are treated quantum mechanically, and the solvent coordinate is treated classically. A relatively large number (e.g., 11) of mixed proton/electron adiabatic states are included in the MDQT simulations. The reaction is initiated on the electronically excited state, and many different dynamical pathways to lower energy stable states are observed. Nonadiabatic effects are shown to play an essential role in determining the rates and mechanisms of photoinduced PCET reactions. This paper differs from previous studies of PCET reactions in that it presents real-time nonadiabatic molecular dynamics simulations of model PCET reactions initiated on an electronically excited state. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    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.
    Type of Medium: Electronic Resource
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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.
    Type of Medium: Electronic Resource
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  • 10
    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.
    Type of Medium: Electronic Resource
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