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(ΔEA) for a molecule A to gain (or lose) an amount of internal energy ΔEA 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 H2 by collision with He is carried out to demonstrate that the FB-IVR provides a good description of quantum effects in P(ΔEA).