Tino G. A. Heijmen^{a}, Tatiana Korona^{b}, Robert Moszynski^{b},
Paul E. S. Wormer^{a}, and Ad van der Avoird^{a}

^{a} Institute of Theoretical Chemistry, NSR Center, University
of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
^{b} Department of Chemistry, University of Warsaw, Pasteura
1, 02-093 Warsaw, Poland

*Ab initio potential-energy surface and rotationally
inelastic integral cross sections of the Ar-CH _{4} complex*

**Abstract**

Symmetry-adapted perturbation theory has been applied to compute the
intermolecular potential-energy surface of the Ar-CH_{4} 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
_{m}=-144.30 cm^{-1} at R_{m}=7.00 bohr, for structures
in which the argon atom approaches the face of the CH_{4} 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 CH_{4} 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.

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