PERTURBATION ANALYSIS OF THE SUPERMOLECULE INTERACTION ENERGY AND THE BASIS SET SUPERPOSITION ERROR

The supermolecule interaction energies at the self-consistent field and the second-order Moller-Plesset perturbation theory levels are analyzed using the polarization approximation perturbation theory of intermolecular interactions. The results for the He-H+ complex show that the perturbation expansion converges and its sum is identical with the supermolecule interaction energy. The counterpoise-corrected supermolecule interaction energies for various complexes in the region of small intermolecular overlap are in excellent agreement with the perturbation results for both the basis sets of monomers and the basis set of the entire complex. Comparison of the results obtained with large and small basis sets for the He-H+ and He-Li+ complexes is used to emphasize the need to remove the basis set superposition error and to demonstrate the correctness of the counterpoise procedure.