Intermolecular energy transfer probabilities from trajectory calculations: A new approach

A new method to calculate intermolecular energy transfer probability density function P(E',E) from trajectory calculations is proposed. The method distinguishes between effective trajectories that contribute to P(E',E) and those with very large impact parameter which do not. The P(E',E) thus found obeys conservation of probability and detailed balance and is independent of the impact parameter. The method is demonstrated for benzene-Ar collisions at various temperatures and internal energies. With this method it is possible to combine ab initio inter and intramolecular potentials with trajectory calculations, obtain P(E',E) and use that in master equation calculations to obtain rate coefficients and populations distributions without resorting to any a priori assumptions and energy transfer models. In addition, the effects of internal energy, temperature and rotations on the average energy transferred are discussed. Global potentials in center-of-mass and minimal distance coordinates which are obtained by averaging 20 000 and 50 000 trajectories are reported. It is shown that Lennard-Jones or ab initio pairwise potentials yield a Buckingham-type global potentials. (C) 1998 American Institute of Physics. [S0021-9606(98)02109-6].