QUANTUM DECOHERENCE IN MIXED QUANTUM-CLASSICAL SYSTEMS - NONADIABATICPROCESSES

We address the issue of quantum decoherence in mixed quantum-classical simulations. We demonstrate that restricting the classical paths to a single path among all the quantum paths affects a coarse graining of the quantum paths. Such coarse graining causes the quantum paths to lo se coherence as the various possible classical paths associated with e ach quantum state diverge. This defines a reduction mapping of the qua ntum density matrix, and we derive a quantum master equation suitable for mixed quantum-classical systems. The equation includes two terms: first, the ordinary quantum Liouvillian which is parametrized by a sin gle classical path, and second, a quantum decoherence term that includ es both a coherence time and length scale which are determined by the dynamics of the classical paths. Model calculations for electronic coh erence loss in nonadiabatic mixed quantum-classical dynamics are prese nted as examples. For a model charge transfer chemical reaction with n onadiabatic transitions, application of the present formulation reveal s that nonadiabaticity is diminished as the decoherence timescale beco mes shorter and adiabatic dynamics are recovered in the limit of rapid decoherence. (C) 1995 American Institute of Physics.