Bartlett Rodney J, Park Young Choon, Bauman Nicholas P, Melnichuk Ann, Ranasinghe Duminda, Ravi Moneesha, Perera Ajith
Quantum Theory Project, University of Florida, Gainesville, Florida 32611-8435, USA.
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Chem Phys. 2020 Dec 21;153(23):234103. doi: 10.1063/5.0029339.
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.
全组态相互作用计算（FCI）最终定义了分子的固有分子轨道描述。其密度矩阵以及从中获得的自然轨道量化了在描述N个相关电子的波函数的参考行列式中具有N个主要占据轨道与剩余虚拟轨道中剩余N个电子数量之间的差异。后者提供了波函数中所需的多行列式特征（MDC）的一种度量。MDC进一步分为弱相关部分和通常称为多参考特征（MRC）的指示更强相关性的部分。如果几个虚拟轨道具有较高的占据数，那么有人可能会认为这些额外的轨道在计算中应被允许发挥更大的作用，就像在多参考方法中，如MCSCF、MR-CI或多参考耦合簇（MR-CC），以提供对FCI的充分近似。然而，与单参考耦合簇计算（SR-CC）及其操作上的单参考运动方程（EOM-CC）扩展的一致性和易用性相比，这些多参考方法中的任何一种都存在使计算复杂化的问题。由于当今大多数“预测性”计算都使用SR-CC理论，评估在簇算符的某种截断下SR-CC的准确性将有助于量化在计算中MRC实际是多大的问题，以及在保持CC/EOM-CC方便的SR计算特性的同时如何减轻它。本文定义了识别MRC情况并有助于评估给定计算可靠性的指标。
