THE TORSIONAL DEPENDENCE OF AN INTERACTION POTENTIAL - THE CH3OH-HE SYSTEM

An interaction potential for the methanol-helium system was calculated, consisting of an SCF part plus a damped dispersion contribution. A double-zeta basis set was used for the methanol, augmented with one set of polarization functions on each atom. The He basis was a (6s/3s) set augmented with two p-functions. The counterpoise method was used to help correct the basis set superposition error. Interaction energies were computed as a function of the relative position of the He (R, theta, phi) as well as of the methanol conformation, described by the single angle-gamma. The SCF potential was found to be relatively more repulsive at the methyl end of the molecule compared to the Billing site-site potential. A phi-shift approximation was found to predict fairly accurately the gamma-dependence of the potential from the staggered conformation interaction alone. The anisotropy of the potential was analyzed using an expansion in products of spherical harmonics in theta, phi and exp(in-gamma) functions, in which it was shown that only n values which are multiples of three appear. The resulting expansion coefficients are those needed for the matrix elements of the potential between two rotational-torsional states in the close-coupling formulation of the scattering problem. It was found that the torsional anisotropy is much weaker than the overall anisotropy and that the phi and gamma-anisotropies are strongly coupled.