Bsse-Corrected Perturbation Theories of Intermolecular Interactions

Abstract

Intermolecular interactions play very important role in chemistry and physics. Several specific quantum chemical methods have been developed for calculating them. The problem of basis set superposition error (BSSE) occurring in these calculations has also been recognized for many years. It is due to the use of finite basis sets: neither the energy of the whole interacting complex (“supermolecule”) nor the energies of the indioovidual constituting molecules (“monomers”) are exact..Calculating the interaction energy as the difference of these quantities, one observes a systematic imbalance, usually leading to too deep minima on the potential surfaces. This is connected with a better description — in the energetic sense — of the individual constituting molecules within the complex, when the extended basis of the whole supermolecule becomes available, than that in the free monomer case. Different approaches have been proposed in the literature to account for this BSSE effect both at the SCF and at the correlated levels of theory. These methods can approximately be divided into two groups. The first, most commonly used approach is the Boys-Bernardi counterpoise (CP) correction scheme[1] in which the supermolecule energy is used without any correction, while the monomer energies are recomputed by using the full dimer basis set for every geometrical arrangement of the complex, so they become distance dependent. We shall discuss that this treatment involves a tacit assumption about the additivity of the BSSE effects. In the other approach, one assumes that the free monomer energies are correct for the given basis set,and the BSSE has to be eliminated in some manner from the supermolecule calculation. Either one omits some terms from the supermolecule energy expression as being due to BSSE[2, 3] or one tries to get rid of it when determining the wavefunction, as used in the so called “Chemical Hamiltonian Approach” (CHA).