A FULL-CI INVESTIGATION INTO THE BSSE PROBLEM

Full-CI calculations have been performed on the four-electron model sy stems He...He and H-2...H-2 in order to compare the a Posteriori Boys- Bernardi (counterpoise) and the a priori CHA ('chemical Hamiltonian ap proach') 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 sligh t 'overcompensation' effect in the van der Waals region, as could be e xpected. The counterpoise-corrected Moller-Plesset MP2 and MP4 perturb ational results are also compared with the full-CI ones and a proposit ion is put forward for a proper definition of the quantity 'BSSE-free interaction energy in a given basis'.