On the adequacy of mixed quantum-classical dynamics in condensed phase systems

A detailed study of mixed quantum-classical approximations for radiative an d nonradiative processes in condensed phase systems is presented and each a pproximation is compared to exact fully quantum mechanical dynamics. The pr oblems are formulated in terms of a quantum mechanical time correlation fun ction, and the corresponding mixed quantum-classical descriptions are obtai ned following the general methodology of Wigner-Kirkwood based on the expan sion in powers of Planck's constant. Illustrative examples of the performan ce and applicability of mixed quantum-classical treatments include vibratio nal and electronic energy relaxation processes, and the calculation of vibr onic absorption spectrum.