SYMMETRY-ADAPTED PERTURBATION-THEORY CALCULATION OF THE HE-HF INTERMOLECULAR POTENTIAL-ENERGY SURFACE

Symmetry-adapted perturbation theory has been applied to compute the HeHF intermolecular potential energy surface for three internuclear distances in the HF subunit. The interaction energy is found to be dominated by the first-order exchange contribution and by the dispersion energy (including the intramonomer correlation effects). However, smaller corrections as the electrostatics, induction, and second-order exchange are found to be nonnegligible, and the final shape of the potential results from a delicate, balance of attractive and repulsive contributions due to the four fundamental intermolecular interactions: electrostatics, exchange, induction, and dispersion. For a broad range of He-HF configurations the theoretical potential agrees very well with the empirical potential of Lovejoy and Nesbitt [C. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 93, 5387 (1990)], which was adjusted to reproduce the near-infrared spectrum of the complex. Our potential has a global minimum of epsilon(m) = -39.68 cm(-1) for the linear He-HF geometry at R(m)= 6.16 bohr, and a secondary minimum of epsilon(m)= -36.13 cm(-1) for the linear He-FH geometry at R(m)=5.59 bohr. These values are in very good agreement with the corresponding empirical results: epsilon(m)= -39.20 cm(-1) and R(m) = 6.17 bohr for the global minimum, and epsilon(m) = -35.12 cm(-1) and R(m) = 5.67 bohr for the secondary minimum.