THE CHEMICAL HAMILTONIAN APPROACH FOR TREATING THE BSSE PROBLEM OF INTERMOLECULAR INTERACTIONS

The two alternative treatments of the BSSE problem of intermolecular i nteractions-the a posteriori counterpoise (CP) correction scheme of Bo ys and Bernardi (BB) and the a priori exclusion of the basic-set super position error (BSSE) by using the ''chemical Hamiltonian approach'' ( CHA)-are discussed. Alternative interpretations of the BB scheme are c ompared and some problems and paradoxes are described which can consis tently be resolved by introducing a hierarchical scheme of N-body CP c orrections. In CHA, one identifies those terms of the Hamiltonian (or Fockian) which cause the BSSE-the same terms cause the energy lowering in the ''ghost orbitals'' calculations; by omitting these terms from the Hamiltonian (Fockian), one can calculate BSSE-free wave functions, by use of which the BSSE-free values of the energy and of other physi cal quantities can be obtained in a conventional manner. CHA was succe ssfully applied at the SCF and DET levels of theory, including systems containing three or more interacting subsystems or even infinite peri odic chains. The pivot full CI calculations performed for small model systems showed the applicability of the approach for treating electron correlation as well. Mast recently, a CHA version of the MP2 theory a nd a second-order BSSE-free intermolecular PT have also been tested; t hey are also applicable for complexes of larger size. It was observed, at any level of theory, that the results given by the a posteriori BB and a priori CHA schemes converge to each other much faster than BSSE disappears from the uncorrected results. (C) 1998 John Wiley & Sons, Inc.