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 i ntermolecular potential-energy surface of the Ar-CH4 complex. The inte raction 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 mi nima 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, T he computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-sta te integral cross sections for rotational excitation of CH4 in collisi ons with argon. The computed cross sections are generally in good agre ement,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, w ith the use of a master equation approach. (C) 1997 American Institute of Physics.