Index of multi-determinantal and multi-reference character in coupled-cluster theory
Index of multi-determinantal and multi-reference character in coupled-cluster theory
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DOI：
10.1063/5.0029339
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发表时间：
2020-12-21
影响因子：
4.4
通讯作者：
Perera, Ajith
中科院分区：
文献类型：
--
作者：
Bartlett, Rodney J.;Park, Young Choon;Perera, Ajith
完整构型相互作用计算（FCI）最终定义了分子的固有分子轨道描述。它的密度矩阵和从中获得的自然轨道量化了在描述 N 相关电子的波函数的参考行列式中具有 N 占主导地位的轨道之间的差异，以及其中有多少 N 电子被留给剩余的虚拟轨道。后者提供了波函数所需的多行列式特征 (MDC) 的测量。 MDC 进一步分为弱相关部分和指示较强相关性的部分，通常称为多参考特征 (MRC)。如果几个虚拟轨道具有很高的占据数，那么人们可能会认为应该允许这些额外的轨道在计算中发挥更大的作用，就像在 MR 方法中一样，例如 MCSCF、MR-CI 或 MR 耦合簇 (MR-CC)，以便为 FCI 提供足够的近似值。然而，与单参考 CC 计算 (SR-CC) 及其可操作的单参考运动方程 (EOM-CC) 扩展的均匀性和易用性相比，这些 MR 方法都存在问题，使计算变得复杂。由于 SR-CC 理论用于当今大多数“预测”计算中，因此在集群算子的某些截断处评估 SR-CC 的准确性将有助于量化计算中 MRC 问题实际有多大，以及如何在保留 CC/EOM-CC 方便的 SR 计算特性的同时减轻该问题。本文定义了识别 MRC 情况并帮助评估给定计算的可靠性的指数。
A full configuration interaction calculation (FCI) ultimately defines the innate molecular orbital description of a molecule. Its density matrix and the natural orbitals obtained from it quantify the difference between having N-dominantly occupied orbitals in a reference determinant for a wavefunction to describe N-correlated electrons and how many of those N-electrons are left to the remaining virtual orbitals. The latter provides a measure of the multi-determinantal character (MDC) required to be in a wavefunction. MDC is further split into a weak correlation part and a part that indicates stronger correlation often called multi-reference character (MRC). If several virtual orbitals have high occupation numbers, then one might argue that these additional orbitals should be allowed to have a larger role in the calculation, as in MR methods, such as MCSCF, MR-CI, or MR-coupled-cluster (MR-CC), to provide adequate approximations toward the FCI. However, there are problems with any of these MR methods that complicate the calculations compared to the uniformity and ease of application of single-reference CC calculations (SR-CC) and their operationally single-reference equation-of-motion (EOM-CC) extensions. As SR-CC theory is used in most of today's "predictive" calculations, an assessment of the accuracy of SR-CC at some truncation of the cluster operator would help to quantify how large an issue MRC actually is in a calculation, and how it might be alleviated while retaining the convenient SR computational character of CC/EOM-CC. This paper defines indices that identify MRC situations and help assess how reliable a given calculation is.