Mpller-Plesset perturbation theory size-extensivity

Mpller-Plesset perturbation theory represents a useful approach to the calculation of size-extensive correlation energies for systems dominated by a single electronic configuration. The MP2 model, in particular, represents a successful compromise between computational cost and accuracy. Higher-order corrections may also be calculated, but it should be emphasized that the M0ller-Plesset series does not converge unconditionally. 

It was suggested in Section 14.2.4 that it is advantageous to express the Mpller-Plesset energy corrections in terms of nested commutators. We shall in this subsection see that such commutator expressions are convenient since they make the energy corrections termwise size-extensive, just like the similarity-transformed formulation of coupled-cluster theory makes the coupled-cluster equations termwise size-extensive, as shown in Section 13.3.2. We first consider the perturbed wave function for two noninteracting systems and then go on to consider the separability of the Mpller-Plesset energies. 

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