A preliminary study of monomer geometry effects in theoretical calculations of the interaction energy for weak molecular complexes

Abstract

An analysis is made of the repercussion that the consideration of the geometrical deformation of the monomers and the basis set superposition error, BSSE, can have on the theoretical calculation of the interaction energy of a hydrogen-bonded complex. The calculations, applied to the (HF) 2 system at the SCF and MP2 levels, were performed using several medium-sized basis sets. The conclusion is that, in order to achieve a good response, the complex's optimum geometry for each basis set has to be used, since otherwise one has to take into account the contribution to the energy coming from the geometrical relaxation experienced by the partner interacting systems. It is also seen that the BSSE must be corrected by eliminating the overestimate that appears in the counterpoise (CP) method of Boys and Bernardi (Mol. Phys., 19 (1970) 53) and that is located in the CP correction of the electronic exchange component of the interaction energy. The conclusion reached are valid at both the SCF and post-SCF levels.