Exact size consistency of multireference Miller-Plesset perturbation theory

Single-reference closed-shell Moller-Plesset perturbation theory is well kn own for its size consistency, a quality that is essential for consistent co mparisons of calculations on molecules of different size. However, it is fa r from obvious whether this quality can be retained in the multireference c ase, in this work it is shown that an exactly size consistently generalizat ion to multireference perturbation theory can be constructed. The central r esult is that the zeroth-order Hamiltonian should be constructed using sepa rate projection operators for each excitation level, i.e., it should contai n no couplings between different excitation levels. (C) 1999 John Wiley & S ons, Inc.