单参考酉多体理论中的连通三体项：迭代和微扰近似
Connected three-body terms in single-reference unitary many-body theories: Iterative and perturbative approximations
摘要：
本文介绍了在酉多体理论中引入连通三体项的各种方法，并以驱动相似性重整化群（DSRG）为例进行了讨论。从最小近似方法开始-线性化的DSRG截断为一体、二体和三体算子[LDSRG（3）] -我们开发了几个近似的LDSRG（3）模型，降低了计算成本。通过微扰分析，我们提出了一族迭代LDSRG（3）- $n$和- $n'$（$n=1,2,3,4$）方法，它们包含LDSRG（3）图的一个子集。在这些变体中，LDSRG（3）-2方案具有与单重、双重和三重耦合簇理论（CCSDT）相同的计算复杂度，但在预测相关能量的准确性方面优于CCSDT。我们还提出并实现了两个微扰三重校正的基础上截断的线性化DSRG的一个和两个身体的运营商与递归二次增广[qDSRG（2）]。由此产生的qDSRG（2）+（T）方法匹配的“金标准”耦合团簇理论与单，双，微扰三重模型的28个封闭壳层原子和小分子的能量的准确性。
英文摘要：
This work introduces various approaches to include connected three-body terms in unitary many-body theories, focusing a representative example on the driven similarity renormalization group (DSRG). Starting from the least approximate method - the linearized DSRG truncated to one-, two-, and three-body operators [LDSRG(3)] - we develop several approximate LDSRG(3) models with reduced computational cost. Through a perturbative analysis, we motivate a family of iterative LDSRG(3)-$n$ and -$n'$ ($n=1,2,3,4$) methods that contain a subset of the LDSRG(3) diagrams. Among these variants, the LDSRG(3)-2 scheme has the same computational complexity of coupled cluster theory with singles, doubles, and triples (CCSDT), but it outperforms CCSDT in the accuracy of the predicted correlation energies. We also propose and implement two perturbative triples corrections based on the linearized DSRG truncated to one- and two-body operators augmented with recursive quadratic commutators [qDSRG(2)]. The resulting qDSRG(2)+(T) approach matches the accuracy of the "gold-standard" coupled cluster theory with singles, doubles, and perturbative triples model on the energetics of twenty-eight closed-shell atoms and small molecules.
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