Application of the forward-backward initial value representation to molecular energy transfer

It is shown how the forward-backward (FB) approximation to the semiclassical initial value representation (IVR) can be used to calculate the probability (or cross section) for molecular energy transfer. Specifically, the probability P(Delta E-A) for a molecule A to gain (or lose) an amount of internal energy Delta E-A by collision with a bath molecule B is given by the Fourier transform of a time correlation function C(t), which is in turn given by a single phase space average over the initial conditions of classical trajectories of the A+B collision system. Application to energy transfer of H-2 by collision with He is carried out to demonstrate that the FB-IVR provides a good description of quantum effects in P(Delta E-A). (C) 1999 American Institute of Physics. [S0021- 9606(99)01847-4].