  We present a scalable single-particle framework to treat electronic
correlation in molecules and materials motivated by Green's function theory. We
derive a size-extensive Brillouin-Wigner perturbation theory from the
single-particle Green's function by introducing the Goldstone self-energy. This
new ground state correlation energy, referred to as Quasi-Particle MP2 theory
(QPMP2), avoids the characteristic divergences present in both second-order
M{\o}ller-Plesset perturbation theory and Coupled Cluster Singles and Doubles
within the strongly correlated regime. We show that the exact ground state
energy and properties of the Hubbard dimer are reproduced by QPMP2 and
demonstrate the advantages of the approach for the six-, eight- and ten-site
Hubbard models where the metal-to-insulator transition is qualitatively
reproduced, contrasting with the complete failure of traditional methods. We
apply this formalism to characteristic strongly correlated molecular systems
and show that QPMP2 provides an efficient, size-consistent regularization of
MP2.
