CONVERGENCE PROPERTIES OF COUPLED HARTREE-FOCK THEORY OF INTERMOLECULAR INTERACTIONS

The interaction between a helium atom and a proton is analyzed using coupled Hartree-Fock perturbation theory in the polarization approximation. It is shown that even for relatively short interatomic separations the perturbation expansion converges and its sum is identical with the supermolecule interaction energy. Similar behavior is observed for molecule-proton complexes although the convergence may be worse and, as an illustration, calculations for H2O-H+ are presented. A detailed analysis of the electrostatic component of the interaction energy between He and H+ shows the failure of a long range multipole expansion in correcting the secondary basis set superposition error. Polarization approximation perturbation theory is also used to analyze interactions for He-Li+ and He-Na+. The results for interatomic distances of 8.0 and 10.0 a0 for both the basis sets of the monomers, and the basis set of the entire complex are in excellent agreement with the counterpoise-corrected interaction energies, and thus show the need to remove the basis set superposition error.