Stein Tamar, Henderson Thomas M, Scuseria Gustavo E
Department of Chemistry, Rice University, Houston, Texas 77251, USA.
J Chem Phys. 2014 Jun 7;140(21):214113. doi: 10.1063/1.4880819.
Coupled cluster theory with single and double excitations accurately describes weak electron correlation but is known to fail in cases of strong static correlation. Fascinatingly, however, pair coupled cluster doubles (p-CCD), a simplified version of the theory limited to pair excitations that preserve the seniority of the reference determinant (i.e., the number of unpaired electrons), has mean field computational 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 static correlation. 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 the coupled cluster doubles (CCD) correlation energy 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 the CCD level is retained at the p-CCD level. Moreover, we explore ways of extending CCD to accurately describe strongly correlated systems.
含单双激发的耦合簇理论能够精确描述弱电子关联，但在强静态关联情况下会失效。然而，有趣的是，对耦合簇双激发（p - CCD），作为该理论的一个简化版本，它仅限于保持参考行列式（即未配对电子数）的 seniority 的对激发，具有平均场计算成本，并且只要定义配对方案的轨道基得到充分优化，它就是配对空间全组态相互作用（FCI）的一个出色近似。在之前的工作中，我们已经表明，在 seniority 为零的 FCI 中对配对方案进行优化会导致对静态关联的非常精确的描述。如果轨道被优化以使 p - CCD 能量稳定，同样的结论也适用于 p - CCD。我们在此用大量例子展示这些结果。我们还使用不同的轨道基探索不同 seniority 扇区对耦合簇双激发（CCD）关联能的贡献。我们考虑了 Hartree - Fock 轨道和 Brueckner 轨道，以及轨道定域化的作用。我们展示了如何对轨道进行配对，以便在 p - CCD 水平上保留 Brueckner 轨道在 CCD 水平上的作用。此外，我们探索扩展 CCD 以精确描述强关联系统的方法。
