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

Symmetry-adapted perturbation theory has been applied to compute the H eHF intermolecular potential energy surface for three internuclear dis tances in the HF subunit. The interaction energy is found to be domina ted by the first-order exchange contribution and by the dispersion ene rgy (including the intramonomer correlation effects). However, smaller corrections as the electrostatics, induction, and second-order exchan ge are found to be nonnegligible, and the final shape of the potential results from a delicate, balance of attractive and repulsive contribu tions due to the four fundamental intermolecular interactions: electro statics, 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. Ne sbitt, J. Chem. Phys. 93, 5387 (1990)], which was adjusted to reproduc e the near-infrared spectrum of the complex. Our potential has a globa l 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 i n very good agreement with the corresponding empirical results: epsilo n(m)= -39.20 cm(-1) and R(m) = 6.17 bohr for the global minimum, and e psilon(m) = -35.12 cm(-1) and R(m) = 5.67 bohr for the secondary minim um.