MANY-BODY SYMMETRY-ADAPTED PERTURBATION-THEORY OF INTERMOLECULAR INTERACTIONS - H2O AND HF DIMERS

A many-body version of the symmetry-adapted perturbation theory is developed for a direct calculation of intermolecular potentials as a sum of the electrostatic, exchange, induction, and dispersion contributions. Since no multipole expansion is used, the obtained interaction energy components are properly dampened at short distance by the charge-overlap (penetration) effects. The influence of the intramonomer correlation is accounted for by the perturbation expansion in terms of the Moller-Plesset type fluctuation potentials W(A) and W(B) for the individual molecules. For the electrostatic and for the dispersion energy, the terms of the zeroth, first, and second order in W(A) + W(B) are considered. In this way, the leading three-particle correlation contribution to the dispersion energy is taken into account. As a test of our method, we have performed calculations of the interaction energy for the water and hydrogen fluoride dimers. Both the geometry and the basis set dependence of the interaction energy components have been investigated. For a comparison, we have also computed the supermolecular interaction energies through the full fourth order of the many-body perturbation theory. On the basis of our results, we predict the association energy for (H2O)2 equal to -4.7 +/- 0.2 kcal/mol in relatively poor agreement with the experimental value of - 5.4 +/- 0.7 kcal/mol, but still within the experimental error bars. For (HF)2, the predicted association energy is -4.2 +/- 0.2 kcal/mol, while the experimental value (corrected by a theoretical zero-point energy) is -4.9 +/- 0.1 kcal/mol.