Exact size consistency of multireference M�ller-Plesset perturbation theory

Abstract

Single-reference closed-shell Moller)Plesset perturbation theory is well known for its size consistency, a quality that is essential for consistent comparisons of calculations on molecules of different size. However, it is far from obvious whether this quality can be retained in the multireference case. In this work it is shown that an exactly size consistently generalization to multireference perturbation theory can be constructed. The central result is that the zeroth-order Hamiltonian should be constructed using separate projection operators for each excitation level, i.e., it should contain no couplings between different excitation levels. Q 1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 549)558, 1999