Extension of a HeH₂ potential energy surface

The CI surface of Tsapline and Kutzelnigg is extended to smaller H2He separations. Defining R as the H2He distance, r as the H2 separation, and γ as the agnel between them, the ab initio values are fit to a Legendre series in cosγ retaining the first three (even) terms with the coefficients given as analytic functions of R and r to facilitate semiclassical scattering computations. The fit is quantitative for 1.0 < r < 1.8 au, 1.8 au < R < ∞ and is expected to extrapolate well in the region 0.5 < r 2.5 au, R {greater-than or approximate} r/2 + 1. © 1977.