Classical energy-sudden calculations of diffusion and viscosity cross sections for atom-diatom interactions

Trajectory calculations are reported in the sudden approximation for the Maxwellian averaged diffusion and viscosity cross sections for He-N 2 , Ar-N 2 and He-CO 2 collisions. Comparison is made with classical infinite-order-sudden (IOS) calculations, also known as the Mason-Monchick approximation, and with various quantal IOS calculations. For the N 2 collisions, the energy-sudden and classical IOS results agree within 1% and differ from the quantal results by less than 2%. For the He-CO 2 collisions, the classical IOS results differ by about 5% from the energy-sudden calculations. An energy-corrected sudden approximation is introduced. This correction is significant only for viscosity-type cross sections, making a 6% difference for Ar-N 2 and He-CO 2 collisions at 300K. Criteria for the validity of the classical IOS approximation are suggested.