Merging multireference perturbation and density-functional theories by means of range separation: Potential curves for Be2, Mg2, and Ca2

A rigorous combination of multireference perturbation theory and density functional theory (DFT) is proposed. Based on a range separation of the regular two-electron Coulomb interaction, it combines a short-range density functional with second-order strongly contracted n-electron valence state perturbation theory (sc-NEVPT2). The huge advantage of the sc-NEVPT2 approach is that the density is unchanged through first order due to a generalized-Brillouin-type theorem so that the computationally cumbersome self-consistency contribution of short-range DFT to the second-order energy correction equals zero. The method yields very promising results for the van der Waals systems Be-2, Mg2, and Ca-2; including the multireference system Be-2.