Synergy between pair coupled cluster doubles and pair density functional theory.

Pair coupled cluster doubles (pCCD) has been recently studied as a method capable of accounting for static correlation with low polynomial cost. We present three combinations of pCCD with Kohn-Sham functionals of the density and on-top pair density (the probability of finding two electrons on top of each other) to add dynamic correlation to pCCD without double counting. With a negligible increase in computational cost, these pCCD+DFT blends greatly improve upon pCCD in the description of typical problems where static and dynamic correlations are both important. We argue that-as a black-box method with low scaling, size-extensivity, size-consistency, and a simple quasidiagonal two-particle density matrix-pCCD is an excellent match for pair density functionals in this type of fusion of multireference wavefunctions with DFT.