Anisotropic repulsive potential energy surfaces from Hartree-Fock calculations for HeCO₂ and HeOCS

Hartree-Fock calculations are presented for the repulsive interactions of He with CO₂ and OCS. The results are well described by parametrizing the anisotropic potential energy surface as a sum over interactions between He and each atom of the molecule. The interaction of He with the oxygen ends of both molecules is almost identical, thereby reducing the number of potential fitting parameters required. The analytic surfaces obtained yield good agreement with pressure broadening measurements, which probe the anisotropy while being independent of the van der Waals attraction. It is suggested that the sum-over-sites parametrization may be useful in constructing semiempirical surfaces that do include the van der Waals attraction. The sum-over-sites parametrization is also particularly well suited to describing the vibrational dependence of the repulsive anisotropy.