BSSE-free second-order intermolecular perturbation theory

A second-order BSSE-free intermolecular perturbation theory is developed starting from the orbitals and orbital energies obtained by solving the monomer SCF problems and treating electron correlation and intermolecular interaction simultaneously. The unperturbed Hamiltonian is defined as the sum of the monomer Fock operators by taking into account the intermolecular overlap. A second-quantized formalism is used in which the creation and annihilation operators pertaining to the non-orthogonal basis are expanded in terms of analogous ones in an auxiliary orthogonal basis. The first-order wavefunction is calculated by using the BSSE-free CHA Hamiltonian and the second-order energy contribution is determined by using the generalized Hylleraas functional recently proposed by one of us. The sample calculations on helium and water dimers show the applicability of the proposed scheme for most different types of intermolecular interaction.