Carter-Fenk Kevin
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15218, United States.
J Phys Chem A. 2025 Aug 7;129(31):7251-7260. doi: 10.1021/acs.jpca.5c03203. Epub 2025 Jun 26.
Linearized Coupled Cluster Doubles (LinCCD) often provides near-singular energies in small-gap systems that exhibit static correlation. This has been attributed to the lack of quadratic terms that typically balance out small energy denominators in the CCD amplitude equations. Herein, I show that exchange contributions to ring and crossed-ring contractions (not small denominators ) cause the divergent behavior of LinCC(S)D approaches. Rather than omitting exchange terms, I recommend a regular and size-consistent method that retains only linear ladder diagrams. As LinCCD and configuration interaction doubles (CID) equations are isomorphic, this also implies that simplification (rather than quadratic extensions) of CID amplitude equations can lead to a size-consistent theory. Linearized ladder CCD (LinLCCD) is robust in statically correlated systems and can be made with a hole-hole approximation. The results presented here show that LinLCCD and its hole-hole approximation can accurately capture energy differences, even outperforming full CCD and CCSD for noncovalent interactions in small-to-medium sized molecules, setting the stage for further adaptations of these approaches that incorporate more dynamical correlation.
线性化耦合簇双激发（LinCCD）在表现出静态关联的小能隙系统中常常给出近奇异的能量。这被归因于缺乏通常能在耦合簇双激发（CCD）振幅方程中平衡小能量分母的二次项。在此，我表明对环和交叉环收缩的交换贡献（而非小分母）导致了LinCC(S)D方法的发散行为。我建议采用一种正则且尺寸一致的方法，该方法只保留线性阶梯图，而不是省略交换项。由于LinCCD和组态相互作用双激发（CID）方程是同构的，这也意味着CID振幅方程的简化（而非二次扩展）可以导致一种尺寸一致的理论。线性化阶梯CCD（LinLCCD）在静态关联系统中是稳健的，并且可以通过空穴 - 空穴近似来实现。这里给出的结果表明，LinLCCD及其空穴 - 空穴近似能够准确捕捉能量差，甚至在中小尺寸分子的非共价相互作用方面优于全CCD和CCSD，为进一步改进这些包含更多动态关联的方法奠定了基础。
