Multi-partitioning quasidegenerate perturbation theory. A new approach to multireference Mّller-Plesset perturbation theory

We propose a new approach to the construction of perturbative expansions for effective Hamiltonians, implying the use of multiple partitioning of the total Hamiltonian. This approach offers the possibility to formulate the state-universal multireference many-body perturbation theory retaining the most important features of conventional Mّller-Plesset expansions. Restricted to the second order, the novel theory gives rise to a simple method of electronic structure calculations which combines a decontracted treatment of model-space components of wavefunctions with the strict size-consistency for complete model spaces and the ability to avoid, or at least attenuate, the intruder-state problem.