MULTI-PARTITIONING QUASI-DEGENERATE PERTURBATION-THEORY - A NEW APPROACH TO MULTIREFERENCE MOLLER-PLESSET PERTURBATION-THEORY

We propose a new approach to the construction of perturbative expansio ns for effective Hamiltonians, implying the use of multiple partitioni ng of the total Hamiltonian. This approach offers the possibility to f ormulate the state-universal multireference many-body perturbation the ory retaining the most important features of conventional Moller-Pless et expansions. Restricted to the second order, the novel theory gives rise to a simple method of electronic structure calculations which com bines a decontracted treatment of model-space components of wavefuncti ons with the strict size-consistency for complete model spaces and the ability to avoid, or at least attenuate, the intruder-state problem.