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  • 1
    Electronic Resource
    Electronic Resource
    SEMICLASSICAL DESCRIPTION OF MOLECULAR DYNAMICS BASED ON INITIAL-VALUE REPRESENTATION METHODS (2004)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Physical Chemistry 55 (2004), S. 299-332 
    Details
    ISSN: 0066-426X
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Chemistry and Pharmacology , Physics
    Notes: Recent progress in the development of semiclassical methods to describe quantum effects in molecular dynamics is reviewed. Focusing on rigorous semiclassical methods that are based on the initial-value representation of the semiclassical propagator, we discuss several promising schemes that have been developed in the past few years to extend the applicability of semiclassical approaches to complex molecular systems. In particular, integral-filtering techniques and forward-backward methods are surveyed. Furthermore, recently proposed approaches that allow the semiclassical description of nonadiabatic molecular dynamics are discussed. The potential and efficiency of these methods is illustrated by selected applications.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.physchem.55.091602.094429
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  • 2
    Electronic Resource
    Electronic Resource
    Forward–backward initial value representation for the calculation of thermal rate constants for reactions in complex molecular systems (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 47-55 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The semiclassical (SC) initial value representation (IVR) provides a potentially practical way for including quantum effects into classical molecular dynamics simulations. The forward–backward (FB) version of the IVR provides an especially attractive way for calculating time correlation functions, in particular the reactive flux correlation function which determines chemical reaction rates. This paper presents a further analysis and development of the FB-IVR approach. Applications show that it is feasible and accurate for a reaction coordinate coupled to up to 40 degrees of freedom. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480560
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  • 3
    Electronic Resource
    Electronic Resource
    Theory of vibrational relaxation processes in resonant collisions of low-energy electrons with large molecules (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6577-6595 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The effect of intramolecular vibrational relaxation in resonant collisions of low-energy electrons with large molecules is investigated. The vibrational relaxation results from the coupling of a few active modes, which are strongly coupled to the electronic degrees of freedom and therefore coherently excited during the formation of the collision complex, to a large number of inactive bath modes. The theory is formulated within the framework of the projection-operator formalism. Starting from a model Hamiltonian which includes all vibrational modes, the bath degrees of freedom are eliminated, using perturbation theory in the system-bath coupling and projection techniques within the bath Hilbert space. The result is an effective-Hamiltonian description of inelastic electron scattering and electron attachment which incorporates vibrational dissipation. In addition, the time-dependent description of resonant electron-molecule scattering is extended to include the possibility of vibrational relaxation in the resonance state. This time-dependent formulation, which is based on the Markovian master equation for the reduced density operator, provides more direct insight into the dynamics of the collision complex. Calculations for simple model systems are performed which yield insight into the characteristic effects of vibrational relaxation on electron scattering and attachment cross sections, as well as on the time-dependent dynamics of shape resonances. In particular, the competition between autodetachment and vibrational relaxation in shape resonances close to threshold is studied in some detail. The inclusion of vibrational relaxation allows us to develop a microscopic dynamical description of the nondissociative capture of low-energy electrons by large molecules. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477310
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  • 4
    Electronic Resource
    Electronic Resource
    Semiclassical description of nonadiabatic quantum dynamics: Application to the S1–S2 conical intersection in pyrazine (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 10282-10292 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently proposed semiclassical approach to the description of nonadiabatic quantum dynamics [G. Stock and M. Thoss, Phys. Rev. Lett. 78, 578 (1997), X. Sun and W. H. Miller, J. Chem. Phys. 106, 916 (1997)] is applied to the S1–S2 conical intersection in pyrazine. This semiclassical method is based on a transformation of discrete quantum variables to continuous variables, thereby bypassing the problem of a classical treatment of discrete quantum degrees of freedom such as electronic states. Extending previous work on small systems, we investigate the applicability of the semiclassical method to larger systems with strong vibronic coupling. To this end, we present results for several pyrazine models of increasing dimensionality and complexity. In particular, we discuss the quality and performance of the semiclassical approach when the number of nuclear degrees of freedom is increased. Comparison with quantum-mechanical calculations and experimental results shows that the semiclassical method is able to describe the ultrafast dynamics in this system. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481668
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  • 5
    Electronic Resource
    Electronic Resource
    Model study of near-threshold photoionization of large molecules: The effect of vibrational relaxation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 3174-3185 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of intramolecular vibrational relaxation in large molecules on near-threshold photoabsorption and photoionization processes is investigated. The vibrational relaxation results from coupling of the initially excited modes to a large number of inactive bath modes. Starting from a model Hamiltonian including all vibrational modes, the bath degrees of freedom are eliminated within the Markov approximation using Hilbert-space projection-operator techniques. Additional Feshbach projection techniques and a threshold expansion of Coulomb Green's function are used to cast the resulting expressions into a numerically tractable form. Predissociation channels are included in a phenomenological manner. The numerical results allow us to study the characteristic effects of vibrational relaxation on absorption and ionization spectra. In particular, the competition between autoionization, predissociation and vibrational relaxation of the ion core is investigated. The suppression of the ionization quantum yield above the lowest ionization threshold as a result of these two decay channels is demonstrated. It is shown that the quenching of autoionization by vibrational relaxation or predissociation can be experimentally distinguished on account of the different scaling behaviour of both processes. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473059
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  • 6
    Electronic Resource
    Electronic Resource
    Self-consistent hybrid approach for complex systems: Application to the spin-boson model with Debye spectral density (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2991-3005 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The self-consistent hybrid approach [H. Wang, M. Thoss, and W. H. Miller, J. Chem. Phys. 115, 2979 (2001), preceding paper] is applied to the spin-boson problem with Debye spectral density as a model for electron-transfer reactions in a solvent exhibiting Debye dielectric relaxation. The population dynamics of the donor and acceptor states in this system is studied for a broad range of parameters, including the adiabatic (slow bath), nonadiabatic (fast bath), as well as the intermediate regime. Based on illustrative examples we discuss the transition from damped coherent dynamics to purely incoherent decay. Using the numerically exact results of the self-consistent hybrid approach as a benchmark, several approximate theories that have been widely used to describe the dynamics in the spin-boson model are tested: the noninteracting blip approximation, the Bloch–Redfield theory, the Smoluchowski-equation treatment of the reaction coordinate (Zusman equations), and the classical path approach (Ehrenfest model). The parameter range where the different methods are applicable are discussed in some detail. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1385562
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  • 7
    Electronic Resource
    Electronic Resource
    Semiclassical description of diffraction and its quenching by the forward–backward version of the initial value representation (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2572-2579 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is able to describe quantum interference/coherence (i.e., diffraction) of particles transmitted by a two-slit potential. (In contrast, the linearized approximation to the SC-IVR, which leads to the classical Wigner model, is unable to do so.) FB-IVR calculations are also used to describe the (partial) quenching of this interference structure (i.e., "de-coherence") when the two-slit potential is coupled to a bath of harmonic oscillators. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337803
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  • 8
    Electronic Resource
    Electronic Resource
    Semiclassical description of quantum coherence effects and their quenching: A forward–backward initial value representation study (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2562-2571 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is used to study quantum coherence effects in the time-dependent probability distribution of an anharmonic vibrational coordinate and its quenching when coupled to a thermal bath. It is shown that the FB-IVR accurately reproduces the detailed quantum coherent structure in the weak coupling regime, and also describes how this coherence is quenched with an increase of the system–bath coupling and/or the bath temperature. Comparisons are made with other approximations and the physical implications are discussed. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337802
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  • 9
    Electronic Resource
    Electronic Resource
    Generalized forward–backward initial value representation for the calculation of correlation functions in complex systems (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9220-9235 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The capability of two different, recently proposed semiclassical (SC) forward–backward (FB) initial value representations (IVR) to describe quantum interference and coherence effects is investigated. It is shown that depending on the way the observable under consideration is represented by unitary operators one can obtain rather different results. Although the FB-IVR based on an integral representation as a rule is capable of describing quantum interference, a closer analysis reveals that it depends on the observable under consideration if all interference that can be described semiclassically is actually included in the calculation. To overcome this problem a new, generalized FB-IVR method (GFB-IVR) is proposed, which combines the capability of the SC-IVR to describe quantum interference effects independent of the observable and the better convergence properties of the FB-IVR. The performance of this new approach is studied in some detail. In particular, it is shown that the GFB-IVR can describe both the coherent and incoherent regime in the dynamics of an anharmonic vibration coupled to a harmonic bath. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1359242
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  • 10
    Electronic Resource
    Electronic Resource
    Systematic convergence in the dynamical hybrid approach for complex systems: A numerically exact methodology (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2979-2990 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient method, the self-consistent hybrid method, is proposed for accurately simulating time-dependent quantum dynamics in complex systems. The method is based on an iterative convergence procedure for a dynamical hybrid approach. In this approach, the overall system is first partitioned into a "core" and a "reservoir" (an initial guess). The former is treated via an accurate quantum mechanical method, namely, the time-dependent multiconfiguration self-consistent field or multiconfiguration time-dependent Hartree approach, and the latter is treated via a more approximate method, e.g., classical mechanics, semiclassical initial value representations, quantum perturbation theories, etc. Next, the number of "core" degrees of freedom, as well as other variational parameters, is systematically increased to achieve numerical convergence for the overall quantum dynamics. The method is applied to two examples of quantum dissipative dynamics in the condensed phase: the spin-boson problem and the electronic resonance decay in the presence of a vibrational bath. It is demonstrated that the method provides a practical way of obtaining accurate quantum dynamical results for complex systems. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1385561
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