Bulik Ireneusz W, Henderson Thomas M, Scuseria Gustavo E
Department of Chemistry, Rice University , Houston, Texas 77005-1892, United States.
Department of Physics and Astronomy, Rice University , Houston, Texas 77005-1892, United States.
J Chem Theory Comput. 2015 Jul 14;11(7):3171-9. doi: 10.1021/acs.jctc.5b00422. Epub 2015 Jun 25.
While restricted single-reference coupled cluster theory truncated to singles and doubles (CCSD) provides very accurate results for weakly correlated systems, it usually fails in the presence of static or strong correlation. This failure is generally attributed to the qualitative breakdown of the reference, and can accordingly be corrected by using a multideterminant reference, including higher-body cluster operators in the ansatz, or allowing symmetry breaking in the reference. None of these solutions are ideal; multireference coupled cluster is not black box, including higher-body cluster operators is computationally demanding, and allowing symmetry breaking leads to the loss of good quantum numbers. It has long been recognized that quasidegeneracies can instead be treated by modifying the coupled cluster ansatz. The recently introduced pair coupled cluster doubles (pCCD) approach is one such example which avoids catastrophic failures and accurately models strong correlations in a symmetry-adapted framework. Here, we generalize pCCD to a singlet-paired coupled cluster model (CCD0) intermediate between coupled cluster doubles and pCCD, yielding a method that possesses the invariances of the former and much of the stability of the latter. Moreover, CCD0 retains the full structure of coupled cluster theory, including a fermionic wave function, antisymmetric cluster amplitudes, and well-defined response equations and density matrices.
虽然截断为单激发和双激发的受限单参考耦合簇理论（CCSD）对于弱相关系统能给出非常精确的结果，但在存在静态或强相关性时通常会失效。这种失效通常归因于参考态的定性崩溃，因此可以通过使用多行列式参考态、在近似式中包含高阶体簇算符或允许参考态中的对称性破缺来校正。这些解决方案都不理想；多参考耦合簇不是黑箱方法，包含高阶体簇算符计算量很大，而允许对称性破缺会导致好量子数的丢失。长期以来人们认识到，准简并性可以通过修改耦合簇近似式来处理。最近引入的对耦合簇双激发（pCCD）方法就是这样一个例子，它避免了灾难性失效，并在对称适应框架中准确地模拟了强相关性。在这里，我们将pCCD推广到耦合簇双激发和pCCD之间的单重态配对耦合簇模型（CCD0），得到一种具有前者不变性和后者大部分稳定性的方法。此外，CCD0保留了耦合簇理论的完整结构，包括费米子波函数、反对称簇振幅以及定义明确的响应方程和密度矩阵。
