Multipartitioning møller-plesset perturbation theory: Size-extensivity at third order and symmetry conservation

Multipartitioning multireference many-body perturbation theory (Zaitevskii and Malrieu, Chem. Phys. Lett. 1995, 233, 597) is investigated with regard to symmetry and size-extensivity.We show that the spin-adapted formulation suffers from spatial symmetry breaking and propose a general symmetry-conserving zero-order Hamiltonian. We analyze size-extensivity of various partitionings at the third order and find that extensivity holds if one-particle quantities in the zero-order Hamiltonian are properly chosen. In particular, third order of the spin-adapted and general symmetry-adapted theory prove to be extensive.