A general efficient implementation of the BSSE-free SCF and MP2 methods based on the chemical Hamiltonian approach

We describe some details related to a new, general, and efficient implementation of the BSSE-free SCF and second-order Møller-Plesset perturbation theories of intermolecular interactions, based on the "Chemical Hamiltonian Approach" (CHA). The program is applicable for both open-shell and closed-shell systems and for an arbitrary number of interacting subsystems. With the new program the CHA method is faster than the usual "counterpoise correction" scheme for single point calculations, especially for clusters consisting of several molecules. The numerical results provided by these conceptually different schemes, however, have again found to be very close to each other. The CHA scheme is particularly good for providing truly BSSE-free MP2 data for intermolecular potentials.