Fitting an a b i n i t i o HF–HF potential surface

The angular dependence of the ab initio rigid-rotor HF–HF surface of Yarkony et al [J. Chem. Phys. 60, 855 (1974)] is fitted with a form appropriate for future quantum and semiclassical scattering calculations, namely a triple series in Legendre polynomials in the molecule–molecule body–fixed angles. The grid of ab initio points is not dense enough to permit a precise fit to the surface, particularly at small center-of-mass separations. Nevertheless, a reasonably accurate fit, which also gives a good description of the HF dimer, can be attained with ∼30 angular terms. The final body-fixed expansion coefficients were transformed to give 20 angular terms in a space-fixed (SF) frame. For the six largest SF terms, exponentials of various arguments as well as inverse powers were used to fit the dependence on the center-of-mass scattering coordinate. A total of 18 parameters were involved. Asymptotically, the interaction potential goes to the expected dipole–dipole plus dipole–quadrupole form. A simple, empirical modification allows the surface to be used for ro-vibrationally inelastic collisions.