Ab initio potential-energy surface and rotationally inelastic integral cross sections of the Ar-CH4 complex

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential-energy surface of the Ar-CH4 complex. The interaction energy, including high-level intramonomer correlation effects, is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has four equivalent minima of epsilon(m) = -144.30 cm(-1) at R-m = 7.00 bohr, for structures in which the argon atom approaches the face of the CH4 tetrahedron, The computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-state integral cross sections for rotational excitation of CH4 in collisions with argon. The computed cross sections are generally in good agreement,with the experimental data [W. B. Chapman et al., J. Chem. Phys. 105, 3497 (1996)]. Some discrepancies for the smallest cross sections can be explained by the influence of sequential collision channels, with the use of a master equation approach. (C) 1997 American Institute of Physics.