Interrelations between the a priori and a posteriori BSSE correction schemes

Abstract

AbstractThe a priori BSSE‐free chemical Hamiltonian approach (CHA) of computing intermolecular interaction energies usually gives results close to those obtained by the a posteriori counterpoise (CP) basis set superposition error (BSSE) correction scheme of Boys and Bernardi. As the basis set improves, the difference between the results of these BSSE‐corrected methods diminishes much faster than BSSE disappears from the uncorrected interaction energies. Here we try to give a possible explanation of this behavior, by considering two different aspects of interrelations between the two schemes. We present a formal perturbation‐theoretical analysis indicating that the behavior can be rationalized on the basis of observing that BSSE and true intermolecular interactions can be considered additive effects under certain conditions, providing formal justification for the tacit additivity assumption inherent in the CP formalism. We also present an analysis of the alternative interpretations of the CP scheme, which allows one to define the CP‐corrected density matrix, permitting calculation of CP‐corrected quantities for electron density and bond orders. Diagonalizing the CP‐corrected density matrix, one can arrive at one‐electron orbitals, the use of which gives energies close to the CHA‐SCF ones. The results indicate that the two conceptually different BSSE correction schemes corroborate each other not only in the sense of giving close numerical results but also at the level of the theoretical analysis. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004