MIXED SEMICLASSICAL-CLASSICAL APPROACHES TO THE DYNAMICS OF COMPLEX MOLECULAR-SYSTEMS

Starting with the semiclassical initial value representation (NR) for the description of a composite molecular system consisting of a ''syst em'' and a ''bath,'' we show a systematic procedure whereby one can re tain the semiclassical IVR description of the ''system''-thus includin g quantum interference and tunneling effects for these degrees of free dom-while neglecting these effects in the ''bath'' degrees of freedom, thus reducing them to a classical description. The zeroth order versi on of this mixed semiclassical-classical model is seen to be the semic lassical equivalent of the Ehrenfest model (closely related to the TDS CF approximation), but higher order versions are also developed that g o beyond this level. of approximation. Two levels of this theory are e xplicitly worked out. Numerical tests on simple but illustrative probl ems are used to discuss the merit of these approaches. (C) 1997 Americ an Institute of Physics.