INTERMOLECULAR POTENTIAL AND ROVIBRATIONAL LEVELS OF AR-HF FROM SYMMETRY-ADAPTED PERTURBATION-THEORY

A two-dimensional intermolecular potential energy surface for Ar-HF has been calculated using the many-body symmetry-adapted perturbation theory (SAPT). The H-F distance was kept constant at its equilibrium value. The interaction energies have been computed using an spdfg-symmetry basis optimized for intermolecular interactions. In addition, the dispersion and induction energies have been calculated in a few progressively larger basis sets to determine the basis set convergence and validity of the asymptotic scaling of those components. Converged results for the dispersion energy have been obtained by using a large basis set containing spdfgh-symmetry orbitals. The ab initio SAPT potential agrees well with the empirical H6(4,3,2) potential of Hutson [J. Chem Phys. 96, 6752 (1992)], including a reasonably similar account of the anisotropy. It predicts an absolute minimum of -207.4 cm(-1) for the linear Ar-HF geometry at an intermolecular separation of 6.53 bohr and a secondary minimum of -111.0 cm(-1) for the linear Ar-FH geometry at an intermolecular separation of 6.36 bohr. The corresponding values for the H6(4,3,2) potential are -211.1 cm(-1) at an intermolecular separation of 6.50 bohr and - 108.8 cm(-1) at an intermolecular separation of 6.38 bohr. Despite this agreement in the overall potentials, the individual components describing different physical effects are quite different in the SAPT and H6(4,3,2) potentials. The SAPT potential has been used to generate rovibrational levels of the complex which were compared to the levels predicted by H6(4,3,2) at the equilibrium separation. The agreement is excellent for stretch-type states (to within 1 cm(-1)), while states corresponding to bending vibrations agree to a few cm(-1). The latter discrepancies are consistent with the differences in anisotropies of the two potentials. (C) 1995 American Institute of Physics.