BSSE-FREE 2ND-ORDER INTERMOLECULAR PERTURBATION-THEORY

A second-order BSSE-free intermolecular perturbation theory is develop ed starting from the orbitals and orbital energies obtained by solving the monomer SCF problems and treating electron correlation and interm olecular interaction simultaneously. The unperturbed Hamiltonian is de fined as the sum of the monomer Fock operators by taking into account the intermolecular overlap. A second-quantized formalism is used in wh ich the creation and annihilation operators pertaining to the non-orth ogonal basis are expanded in terms of analogous ones in an auxiliary o rthogonal basis. The first-order wavefunction is calculated by using t he BSSE-free CHA Hamiltonian and the second-order energy contribution is determined by using the generalized Hylleraas functional recently p roposed by one of us. The sample calculations on helium and water dime rs show the applicability of the proposed scheme for most different ty pes of intermolecular interaction.