Considerations in constructing a multireference second-order perturbation theory

For a closed shell ground state with no neardegeneracy effects, a Hartree-Fock calculation followed by second-order many-body perturbation theory, MBPT2, provides reasonably accurate results at low cost in computer time. Hence, for a multireference second-order perturbation theory, we would like a method that reduces to the MBPT2 method for a single closed shell reference state and which retains the important features of MBPT2 for multireference cases. At the same time, we seek a method that overcomes the major defect of MBPT2-it does not apply to excited states or nearly degenerate ground states. Consequently, it does not give uniform accuracy over much of the ground state potential energy surface. MBPT2 has many special properties, some of which must be lost in any generalization, so it is important to realize which features are essential for accuracy and low cost. Among the attributes of closed-shell single reference MBPT2, that are sometimes claimed to lead to its success, are the following. ( 1) There exists a “one-body” operator, Ho, given as the sum of the Fock operators for each electron, F(i) . The zeroth-order reference state is an eigenfunction of this onebody operator’