The potential energy surfaces for Ar–CO and He–CO were calculated at the fourth order Mo/ller–Plesset perturbation theory and analyzed using perturbation theory of intermolecular forces. Both the complexes reveal only one minimum related to the approximately T-shaped geometry. For Ar–CO, our best ab initio estimates of Re and De are 3.70 Å and 496 μhartrees, respectively, and the optimal angle Rg–com–O is about 80°. For He–CO, our best Re and De are 3.4 Å and 100 μhartrees, respectively, at the optimal angle Rg–com–O of 70°. Our geometrical parameters agree very well with the experimental data. Our ab initio well depths are estimated to be within ±5% in error and are expected to be the most accurate in the literature so far. The De values were obtained with extended basis sets which included bond functions. Basis set effects on the dispersion and electrostatic correlation terms that are caused by bond functions were also analyzed. Both complexes are bound by dispersion forces, but the anisotropy of the interaction is determined by the exchange repulsion component. This anisotropy may be interpreted in terms of the relative concentrations and depletions in the diffuse region of the CO charge density. The electrostatic and induction effects proved to be negligible.

The combination of supermolecular M011er-Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of ArHCt. Two minima have been found, a primary for collinear Ar-HCl and a secondary for collinear Ar-CIH. The depths of these minima are about 12% below the empirical estimates, due to basis set unsaturation of the dispersion contribution. The Ar-HCI structure is favored by the induction and dispersion terms whereas Ar-ClH by the exchange-repulsion term. The total ab initio potential, as well as its components, are compared with related terms of recent Hutson's H6( 3) potential [J. Chern. Phys. 89, 4550 ( 1988)] and the anisotropy of interaction is analyzed. It is concluded that the one-center multipole expansions of induction and dispersion contributions do not reproduce the correct anisotropy of induction and dispersion terms. Ab initio estimates of three-body effects in the Ar 2 HCl complex are also discussed.

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