Browsing by Author "Stein, Tamar"

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    Seniority zero pair coupled cluster doubles theory
    (AIP Publishing, 2014) Stein, Tamar; Henderson, Thomas M.; Scuseria, Gustavo E.
    Coupled clusterﾠtheoryﾠwith single and doubleﾠexcitationsﾠaccurately describes weak electronﾠcorrelationﾠbut is known to fail in cases of strong staticﾠcorrelation.ﾠFascinatingly, however, pairﾠcoupled clusterﾠdoubles (p-CCD), a simplified version of theﾠtheoryﾠlimited to pairﾠexcitationsﾠthat preserve the seniority of the reference determinant (i.e.,ﾠthe number of unpaired electrons), hasﾠmean fieldﾠcomputational 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 staticﾠcorrelation.ﾠThe 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 theﾠcoupled clusterﾠdoublesﾠ(CCD)ﾠcorrelationﾠenergy 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 theﾠCCDﾠlevel is retained at the p-CCD level. Moreover, we explore ways of extendingﾠCCDﾠto accurately describe strongly correlated systems.
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    Seniority-based coupled cluster theory
    (AIP Publishing, 2014) Henderson, Thomas M.; Bulik, Ireneusz W.; Stein, Tamar; Scuseria, Gustavo E.
    Doubly occupied configuration interaction (DOCI) with optimized orbitals often accurately describes strong correlations while working in a Hilbert space much smaller than that needed for full configuration interaction. However, the scaling of such calculations remains combinatorial with system size. Pair coupled cluster doubles (pCCD) is very successful in reproducing DOCI energetically, but can do so with low polynomial scaling (N3, disregarding the two-electron integral transformation from atomic to molecular orbitals). We show here several examples illustrating the success of pCCD in reproducing both the DOCI energy and wave function and show how this success frequently comes about. What DOCI and pCCD lack are an effective treatment of dynamic correlations, which we here add by including higher-seniority cluster amplitudes which are excluded from pCCD. This frozen pair coupled cluster approach is comparable in cost to traditional closed-shell coupled cluster methods with results that are competitive for weakly correlated systems and often superior for the description of strongly correlated systems.