Multi-state multi-reference Møller-Plesset second-order perturbation theory for molecular calculations

This work presents multi-state multi-reference Moller–Plesset second-order perturbation theory as a variant of multi-reference perturbation theory to treat electron correlation in molecules. An effective Hamiltonian is constructed from the first-order wave operator to treat several strongly interacting electronic states simultaneously. The wave operator is obtained by solving the generalized Bloch equation within the first-order interaction space using a multi-partitioning of the Hamiltonian based on multi-reference Moller–Plesset second-order perturbation theory. The corresponding zeroth-order Hamiltonians are nondiagonal. To reduce the computational effort that arises from the nondiagonal generalized Fock operator, a selection procedure is used that divides the configurations of the first-order interaction space into two sets based on the strength of the interaction with the reference space. In the weaker interacting set, only the projected diagonal part of the zeroth-order Hamiltonian is taken into account. The justification of the approach is demonstrated in two examples: the mixing of valence Rydberg states in ethylene, and the avoided crossing of neutral and ionic potential curves in LiF. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006