Diffusion equation coefficients for gas-phase energy transfer

Collisional energy transfer in gas-phase systems is generally modelled using a master equation, which requires a detailed model for the collisional energy transition density. The master equation can be approximated by a diffusion equation which is easier to treat numerically and only requires the specification of drift and diffusion coefficients. Published methods for calculating these coefficients suffer from numerical inaccuracy at high energies. The high energy limits for the drift and diffusion coefficients are derived for the exponential down model and the approach to this limit is examined. At moderately high energies (such as energies above the reaction threshold) the coefficients depend only weakly on the energy. The method is applied to the methyl radical association reaction.