LOCALIZED BOND ORBITALS AND CORRELATION PROBLEM .I. PERTURBATION CALCULATION OF GROUND STATE ENERGY

A method is proposed to go beyond the SCF result in the calculation of the ground state energies without any variational procedure. One chooses a set of reasonable bonding and antibonding orbitals localized on the chemical bonds. The bonding orbitals are used to built a fully localized determinant. The basis of excited states is built using the antibonding orbitals. One calculates the lower eigenvalue of the CI matrix in this basis by a Rayleigh-Schrödinger expansion. The conceptual and practical advantages of the method are discussed, and the perturbation series is specified in order to satisfy the linked cluster theorem conditions and to retain the advantages of the Epstein-Nesbet partition of the Hamiltonian. © 1969 Springer-Verlag.