The chemical Hamiltonian approach for treating the BSSE problem of intermolecular interactions

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The two alternative treatments of the BSSE problem of intermolecular interactions—the a posteriori counterpoise (CP) correction scheme of Boys and Bernardi (BB) and the a priori exclusion of the basic-set superposition error (BSSE) by using the “chemical Hamiltonian approach” (CHA)—are discussed. Alternative interpretations of the BB scheme are compared and some problems and paradoxes are described which can consistently be resolved by introducing a hierarchical scheme of N-body CP corrections. 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 physical quantities can be obtained in a conventional manner. CHA was successfully applied at the SCF and DFT levels of theory, including systems containing three or more interacting subsystems or even infinite periodic chains. The pivot full CI calculations performed for small model systems showed the applicability of the approach for treating electron correlation as well. Most recently, a CHA version of the MP2 theory and a second-order BSSE-free intermolecular PT have also been tested; they 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. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 41–63, 1998