Seniority zero pair coupled cluster doubles theory

Coupled clustertheorywith single and doubleexcitationsaccurately describes weak electroncorrelationbut is known to fail in cases of strong staticcorrelation.Fascinatingly, however, paircoupled clusterdoubles (p-CCD), a simplified version of thetheorylimited to pairexcitationsthat preserve the seniority of the reference determinant (i.e.,the number of unpaired electrons), hasmean fieldcomputational cost and is an excellent approximation to the full configuration interaction (FCI) of the paired space provided that the orbital basis defining the pairing scheme is adequately optimized. In previous work, we have shown that optimization of the pairing scheme in the seniority zero FCI leads to a very accurate description of staticcorrelation.The same conclusion extends to p-CCD if the orbitals are optimized to make the p-CCD energy stationary. We here demonstrate these results with numerous examples. We also explore the contributions of different seniority sectors to thecoupled clusterdoubles(CCD)correlationenergy using different orbital bases. We consider both Hartree-Fock and Brueckner orbitals, and the role of orbital localization. We show how one can pair the orbitals so that the role of the Brueckner orbitals at theCCDlevel is retained at the p-CCD level. Moreover, we explore ways of extendingCCDto accurately describe strongly correlated systems