Analytic fit of the He-H2energy surface in the repulsive region

Single-configuration Hartree-Fock calculations are reported on the He${\mathrm{H}}_2$ triatomicmolecular system in the range of small He-${\mathrm{H}}_2$ separation, $R$, to complement earlier studies, so that the entire energy surface of interest to He on ${\mathrm{H}}_2$ collisions in the low-keV collision-energy regime has now been determined. It is found that strong repulsive three-body polarization forces exist which dominate the physics of this collision-energy regime. Because a Legendre-polynomial expansion in the angular variable is found to be useless in this region of the energy surface, a new parametrization is introduced which includes both binary and three-body terms. The three-body terms are needed because the electron distribution in ${\mathrm{H}}_2$ is different from the sum of the distributions of two individual H atoms.