A quasi-classical approximation for autocorrelation functions

A quasi-classical approximation is presented for the calculation of time autocorrelation functions of properties of bound molecular systems. In this treatment, classical motion is restricted to quantized invariant tori on a discretized energy shell. The dynamics on these tori is obtained by the adiabatic switching technique. When applied to the Henon-Heiles system, the quasi-classical approximation is sometimes found to yield significantly better agreement with the quantum results than that obtained from conventional classical calculations.