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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2995-3002 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Second-order quantized Hamiltonian dynamics (QHD-2) is mapped onto classical mechanics by doubling the dimensionality. The mapping establishes the classical canonical structure for QHD-2 and permits its application to problems showing zero-point energy and tunneling via a standard molecular dynamics simulation, without modifying the simulation algorithms, by introducing new potentials for the extra variables. The mapping is applied to the family of Gaussian approximations, including frozen and thawed Gaussians, which are special cases of QHD-2. The mapping simplifies numerous applications of Gaussians to simulations of spectral intensities and line shapes, nonadiabatic and other quantum phenomena. The analysis shows that frozen Gaussians conserve the total energy, while thawed Gaussians do not, unless an additional term is introduced to the equation of motion for the thawed Gaussian momentum. The classical mapping of QHD-2 is illustrated by tunneling and zero-point energy effects in the harmonic oscillator, cubic and double-well potential, and the Morse oscillator representing the O–H stretch of the SPC-F water model. © 2002 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 113 (2000), S. 6557-6565 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Hamilton approach to classical dynamics is extended to incorporate quantum effects. Quantization of the Hamilton equations of motion results in a hierarchy of equations that are equivalent to quantum mechanics in the Heisenberg form. Closure of the hierarchy gives approximations to the exact quantum dynamics. A specific dynamics algorithm is presented and tested against model applications that exhibit tunneling and zero point motion effects. The quantized Hamilton approach is found accurate, consistent, flexible, and computationally very efficient. © 2000 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 109 (1998), S. 6390-6395 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulation has been used to explore the nature of solvation dynamics for an excess electron in methanol and in water. We perform the analysis within the linear response theory and show that nonlinear corrections are small in both cases. The response function characterizing solvent relaxation after electron photoexcitation and that following the subsequent nonradiative transition are modeled and found to behave very similarly in methanol, in contrast to water. For methanol, each is comprised of an extremely short Gaussian inertial component of small amplitude and a bi-exponential diffusive decay. A relatively fast ∼1 ps exponential accounts for approximately half of the solvent relaxation and is followed by a slower ∼7 ps relaxation of comparable magnitude, a solvation response that is rather similar to that reported previously for relatively large molecules in methanol. Spectral densities of energy gap fluctuations for the equilibrium ground and excited state trajectories show that translational motion dominates solvation. Relaxational processes in methanol have been compared with the results for water. In contrast to methanol, librational motions of solvent molecules significantly influence aqueous solvation dynamics, especially following excited state decay. This difference is reflected in the relaxational processes, which are an order of magnitude slower in methanol than in water. © 1998 American Institute of Physics.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we explore in detail the way in which quantum decoherence is treated in different mixed quantum-classical molecular dynamics algorithms. The quantum decoherence time proves to be a key ingredient in the production of accurate nonadiabatic dynamics from computer simulations. Based on a short time expansion to a semiclassical golden rule expression due to Neria and Nitzan [J. Chem. Phys. 99, 1109 (1993)], we develop a new computationally efficient method for estimating the decay of quantum coherence in condensed phase molecular simulations. Using the hydrated electron as an example, application of this method finds that quantum decoherence times are on the order of a few femtoseconds for condensed phase chemical systems and that they play a direct role in determining nonadiabatic transition rates. The decay of quantum coherence for the solvated electron is found to take ≈50% longer in D2O than in H2O, providing a rationalization for a long standing puzzle concerning the lack of experimentally observed isotope effect on the nonadiabatic transition rate: Although the nonadiabatic coupling is smaller in D2O due to smaller nuclear velocities, the smaller coupling in D2O adds coherently for a longer time than in H2O, leading to nearly identical nonadiabatic transition rates. The implications of this isotope dependence of the nonadiabatic transition rate on changes in the quantum decoherence time for electron transfer and other important chemical reactions are discussed. © 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 107 (1997), S. 825-834 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of many degree of freedom systems are often comprised of classical evolutions on quantum adiabatic energy surfaces with intermittent instantaneous hops from one surface to another. However, since quantum transitions are inherently nonadiabatic processes, the adiabatic approximation underlying the classical equations of motion does not hold in the regions where quantum transitions take place, and the restriction to classical trajectories for adiabatic quantum states is an approximation. Alternatives which employ classical paths that account more fully for nonadiabaticity can be computationally expensive and algorithmically complicated. Here, we propose a new method, which combines the surface hopping idea with the mean-field approximation for classical paths. Applied to three test systems, the method is shown to outperform the methods based on an adiabatic force without significant extra effort. This makes it an appealing alternative for modeling complex quantum–classical processes. © 1997 American Institute of Physics.
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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. 5863-5878 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The impact of quantum decoherence and zero point motion on non-adiabatic transition rates in condensed matter systems is studied in relation to non-adiabatic (NA) molecular dynamics (MD) techniques. Both effects, and decoherence in particular, strongly influence the transition rate, while neither is accounted for by straightforward quantum-classical approaches. Quantum corrections to the quantum-classical results are rigorously introduced based on Kubo's generating function formulation of Fermi's Golden rule and the frozen Gaussian approximation for the nuclear wave functions. The development provides a one-to-one correspondence between the decoherence function and the Franck–Condon factor. The decoherence function defined in this paper corrects an error in our previous work [J. Chem. Phys. 104, 5942 (1996)]. The relationship between the short time approach and the real time NA MD is investigated and a specific prescription for incorporating quantum decoherence into NA simulations is given. The proposed scheme is applied to the hydrated electron. The rate of excited state non-radiative relaxation is found to be very sensitive to the decoherence time. Quantum coherence decays about 50% faster in H2O than in D2O, providing a theoretical rationalization for the lack of experimentally observed solvent isotope effect on the relaxation rate. Microscopic analysis of solvent mode contributions to the coherence decay shows that librational degrees of freedom are primarily responsible, due to the strong coupling between the electron and molecular rotations and to the small widths of the wave packets describing these modes. Zero point motion of the O–H bonds decreases the life time of the excited state of the hydrated electron by a factor of 1.3–1.5. The implications of the use of short time approximations for the NA transition rate and for the evolution of the nuclear wave functions are considered. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An analytical model for the nonlinear behavior of the electro-optic (EO) coefficient in chromophore–polymeric materials is developed. The sharp decline of the EO coefficient above a threshold chromophore concentration is attributed to a second order phase transition transforming the chromophore dipolar system into an antiferroelectric state. The rise of antiferroelectric correlations between chromophore dipoles deteriorates the efficiency of the poling process aimed at achieving a noncentrosymmetric chromophore ordering by application of an electric field. The location of the phase transition and the magnitude of the EO coefficient are investigated as functions of molecular and thermodynamic parameters. Particularly remarkable observations are made regarding the dependence of the EO coefficient on the macroscopic shape of samples used for poling. Slab shaped samples that are common in practice are least efficient for the poling process. Any degree of sample elongation in the direction of the poling field shifts the antiferroelectric phase transition towards higher chromophore concentrations and radically increases the maximum value of the EO coefficient. The theory is applied to two chromophore systems that are typical of materials used in EO devices. Fine agreement with the experimental data is achieved with little adjustment. © 2002 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 8366-8377 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A stochastic mean-field (SMF) approach to nonadiabatic molecular simulations is introduced. Based on the quantum-classical mean-field approximation, SMF extents the classical model of the environment to incorporate its quantum properties. SMF differs from the ordinary mean-field method by the presence of additional terms in the Schrödinger equation that are due to the system-environment interaction. SMF resolves the two major drawbacks of mixed quantum-classical models. First, decoherence effects in the quantum subsystem are rigorously included. Present in all open systems, decoherence is crucial for nonadiabatic transitions taking place in condensed media. Second, the correct branching of the quantum-classical trajectories is achieved. In earlier approaches, the correct branching of the trajectories was attained via ad hoc surface hopping procedures, which experienced the hop rejection problem and could produce unfavorable classical trajectories in regions of nonadiabatic transitions depending on the quantum basis. It is shown that the correct branching of the trajectories is a direct consequence of decoherence. It is argued that the hop rejection problem disappears in SMF. The decoherence operator is discussed in detail, and the properties of the SMF method are illustrated with model simulations. © 1999 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 111 (1999), S. 7818-7827 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel ab initio method is presented for characterization of electron transfer (ET). The method utilizes perturbed ground state (PGS) properties of the ET systems in order to evaluate the electron donor–acceptor coupling and the donor–acceptor energy splitting. Since no excited states are involved in calculations, density functional implementation of the method provides an efficient way to include electron correlation effects for ET in large chemical systems. The PGS method is applied to two model systems and is compared with high-level ab initio results. The PGS method performs very well for the test systems. The method is more general than traditional techniques, providing both the ET coupling and the donor–acceptor energy splitting. © 1999 American Institute of Physics.
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