在酉耦合簇双激发理论中校正缺失单激发的一种有吸引力的方法
An attractive way to correct for missing singles excitations in unitary coupled cluster doubles theory
AI总结：
本文在 UCC 框架下利用 MBPT 低阶能量修正恢复 UCCD 缺失的单激发，提出 UCCD[4S] 与 UCCD[6S]，使结果达到 UCCSD 质量。
摘要：
仅基于双激发的耦合簇方法相对“廉价”，是颇具吸引力的模型化学方法，因为它们通常能够捕捉大部分动态电子相关效应。这种近似的代价在于，被忽略激发（尤其是单激发）的影响可能相当显著。我们使用标准的和电子对限制的 T2 算符来定义两种形式的酉耦合簇双激发（UCCD）方法，并在酉耦合簇（UCC）形式体系内研究通过多体微扰理论（MBPT）的低阶修正能够在多大程度上恢复缺失的单激发。我们的分析包括推导有限阶 UCC 能量泛函，并以此为基础定义对遗漏单激发的微扰估计。由此产生了新颖的 UCCD[4S] 和 UCCD[6S] 方法，它们分别考虑 MBPT 中至四阶和六阶的缺失单激发能量修正。我们还将相同方法应用于电子对限制拟设，但改进仅为边际性的。研究结果表明，用这些事后微扰修正增强 UCCD，可带来达到 UCCSD 质量的结果。
英文摘要：
Coupled cluster methods based exclusively on double excitations are comparatively "cheap" and interesting model chemistries, as they are typically able to capture the bulk of the dynamical electron correlation effects. The trade-off in such approximations is that the effect of neglected excitations, particularly single excitations, can be considerable. Using standard and electron pair-restricted $T_2$ operators to define two flavors of unitary coupled cluster doubles (UCCD) methods, we investigate the extent in which missing single excitations can be recovered from low-order corrections in many-body perturbation theory (MBPT) within the unitary coupled cluster (UCC) formalism. Our analysis includes the derivations of finite-order, UCC energy functionals which are used as a basis to define perturbative estimates of missed single excitations. This leads to the novel UCCD[4S] and UCCD[6S] methods, which consider energy corrections for missing singles excitations through fourth- and sixth-order in MBPT, respectively. We also apply the same methodology to the electron pair-restricted ansatz, but the improvements are only marginal. Our findings show that augmenting UCCD with these post hoc perturbative corrections can lead to UCCSD-quality results.
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