A comparison of close coupling and coupled states for low energy spherical top–atom collisions: Ar–CH4

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Abstract
Accurate calculations were made for collisions of ground state methane with argon atoms. Methane was treated as a rigid rotor and an empirical interaction potential was used. The calculations were performed at low energies for which only a few rotor states were accessible. The collision energies were of the same magnitude as the attractive well depth of the interaction potential and rainbow oscillations were seen. The coupled states approximation, the statistical coupled states approximation, and the truncated coupled states approximation were then compared with the exact results. These approximations gave good agreement for elastic integral and differential cross sections, but at these low energies, somewhat worse agreement for inelastic cross sections than are found with atom–diatomic collisions. In all cases the small angle differential cross sections were good in the coupled states approximation. A clear failure of the coupled states approximation was found. For the model potential used in the present calculations, a rotational transition which is not forbidden in the close coupling scheme is forbidden in the coupled states approximation. This transition is not energetically accessible in the present calculations.