A full-CI investigation into the BSSE problem

Abstract

Full-CI calculations have been performed on the four-electron model systems He…He and H 2…H 2 in order to compare the a posteriori Boys-Bernardi (counterpoise) and the a priori CHA (‘chemical Hamiltonian approach’) schemes for correcting BSSE (‘basis set superposition error’) at the electron correlation level. The results indicate that the CHA scheme performs excellently for correlated systems: analogously to the previous SCF calculations, the Boys-Bernardi and the CHA schemes lead to similar results. The correlation contributions to the interaction energy are especially close; the Boys-Bernardi scheme exhibits a slight ‘overcompensation’ effect in the van der Waals region, as could be expected. The counterpoise-corrected Møller-Plesset MP2 and MP4 perturbational results are also compared with the full-CI ones and a proposition is put forward for a proper definition of the quantity ‘BSSE-free interaction energy in a given basis’.