EXACT PERTURBATION TREATMENT OF THE BASIS SET SUPERPOSITION CORRECTION

It is shown that a complete removal of the basis set superposition effect on the interaction properties of a composite system can be understood in terms of constraints imposed on its wave function. This interpretation of the basis set superposition effect is used to build variation methods which account for the constraints imposed by the incompleteness of available solutions for the constituent subsystems. A perturbation analysis of the resulting constrained equations provides an exact method for the evaluation of the basis set superposition contribution to interaction properties. The corresponding perturbation correction is explicitly evaluated for the Hartree-Fock approximation and discussed at the level of different correlated methods.