On the importance of the fragment relaxation energy terms in the estimation of the basis set superposition error correction to the intermolecular interaction energy

The inclusion of the fragment relaxation energy terms in the estimation of the basis set superposition error (BSSE) correction to the interaction energy is necessary in order to ensure formal convergence to the uncorrected result at the complete basis set (CBS) limit. The problems associated with their omission are demonstrated for F-(H2O), Cl-(H2O), and (H2O)(2) especially when very large basis sets are used. The family of correlation consistent basis sets allows for a heuristic extrapolation of both uncorrected and BSSE-corrected electronic energy differences of the three complexes to the MP2 CBS limits of -27.1, -15.1, and -4.9 kcal/mol respectively. (C) 1996 American Institute of Physics.