809 research outputs found
Monte Carlo sampling of Wigner functions and surface hopping quantum dynamics
The article addresses the achievable accuracy for a Monte Carlo sampling of Wigner functions in combination with a surface hopping algorithm for non-adiabatic quantum dynamics. The approximation of Wigner functions is realized by an adaption of the Metropolis algorithm for real-valued functions with disconnected support. The integration, which is necessary for computing values of the Wigner function, uses importance sampling with a Gaussian weight function. The numerical experiments agree with theoretical considerations and show an error of 2–3%
Surface-hopping dynamics and decoherence with quantum equilibrium structure
In open quantum systems decoherence occurs through interaction of a quantum
subsystem with its environment. The computation of expectation values requires
a knowledge of the quantum dynamics of operators and sampling from initial
states of the density matrix describing the subsystem and bath. We consider
situations where the quantum evolution can be approximated by quantum-classical
Liouville dynamics and examine the circumstances under which the evolution can
be reduced to surface-hopping dynamics, where the evolution consists of
trajectory segments evolving exclusively on single adiabatic surfaces, with
probabilistic hops between these surfaces. The justification for the reduction
depends on the validity of a Markovian approximation on a bath averaged memory
kernel that accounts for quantum coherence in the system. We show that such a
reduction is often possible when initial sampling is from either the quantum or
classical bath initial distributions. If the average is taken only over the
quantum dispersion that broadens the classical distribution, then such a
reduction is not always possible.Comment: 11, pages, 8 figure
Single switch surface hopping for a model of pyrazine
The single switch trajectory surface hopping algorithm is tested for numerical simulations of a two-state three-mode model for the internal conversion of pyrazine through a conical intersection of potential energy surfaces. The algorithm is compared to two other surface hopping approaches, namely, Tully’s method of the fewest switches [J. Chem. Phys. 93, 1061 (1990)] and the method by Voronin et al. [J. Phys. Chem. A 102, 6057 (1998)] . The single switch algorithm achieves the most accurate results. Replacing its deterministic nonadiabatic branching condition by a probabilistic accept-reject criterion, one obtains the method of Voronin et al. without momentum adjustment. This probabilistic version of the single switch approach outperforms the considered algorithms in terms of accuracy, memory requirement, and runtime
Ab initio statistical mechanics of surface adsorption and desorption: I. HO on MgO (001) at low coverage
We present a general computational scheme based on molecular dynamics (m.d.)
simulation for calculating the chemical potential of adsorbed molecules in
thermal equilibrium on the surface of a material. The scheme is based on the
calculation of the mean force in m.d. simulations in which the height of a
chosen molecule above the surface is constrained, and subsequent integration of
the mean force to obtain the potential of mean force and hence the chemical
potential. The scheme is valid at any coverage and temperature, so that in
principle it allows the calculation of the chemical potential as a function of
coverage and temperature. It avoids all statistical mechanical approximations,
except for the use of classical statistical mechanics for the nuclei, and
assumes nothing in advance about the adsorption sites. From the chemical
potential, the absolute desorption rate of the molecules can be computed,
provided the equilibration rate on the surface is faster than the desorption
rate. We apply the theory by {\em ab initio} m.d. simulation to the case of
HO on MgO (001) in the low-coverage limit, using the Perdew-Burke-Ernzerhof
(PBE) form of exchange-correlation. The calculations yield an {\em ab initio}
value of the Polanyi-Wigner frequency prefactor, which is more than two orders
of magnitude greater than the value of s often assumed in the
past. Provisional comparison with experiment suggests that the PBE adsorption
energy may be too low, but the extension of the calculations to higher
coverages is needed before firm conclusions can be drawn. The possibility of
including quantum nuclear effects by using path-integral simulations is noted.Comment: 11 pages + 10 figure
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