The HeCl2 potential: Atom–atom and ab   initio compared to experiment

Two forms for the HeCl2 potential are compared to the available experimental data. First, an atom–atom form that incorporates the recently measured anisotropic He–Cl potential is used. The anisotropy of this potential is slight, and its strengths and weaknesses are similar to previous potentials in which the He–Cl interaction was treated as isotropic. In particular, the fit to the scattering data is poor. Second, a fit to ab initio points calculated using Mo/ller–Plesset perturbation theory to fourth order was performed. The resulting potential is much more anisotropic than any potential previously proposed and tested for HeCl2. This potential fits the rotationally resolved excitation spectra as well as do previous empirical potentials, and is consistent with certain features of the total differential scattering data with which previous potentials were not. Although the ab initio potential has a global minimum in the linear configuration, the probability distribution of the ground vibrational level still maximizes in the perpendicular configuration, accounting for the good fit to the rotationally resolved spectrum. We conclude that noble gas–halogen potentials are much more anisotropic than previously believed, and we suggest several experiments that could help to confirm this anisotropy.