Diagrammatic complete active space perturbation theory

Abstract

A second-order perturbation theory is formulated, called diagrammatic complete active space perturbation theory (diagrammatic CASPT2), that is based on multireference perturbation theory (MRPT) with one- and two-body energy denominator shifts. The energy denominator shifts are chosen so that they avoid the influence of intruder states by enlarging potentially problematic energy denominators and define the wave operator to be similar to the one used in the state specific formalisms, like CASPT2. The wave operator for this method is more efficient to compute than the state specific formalisms, but is expected to yield similar results. An approximation is suggested that eliminates the need to diagonalize the effective Hamiltonian.