School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Institute of Theoretical and Computational Chemistry, Nanjing University, Nanjing 210093, People's Republic of China.
J Chem Phys. 2010 Mar 21;132(11):114115. doi: 10.1063/1.3359851.
An approximate coupled cluster singles, doubles, and triples (CCSDT) method based on the unrestricted Hartree-Fock (UHF) reference, in which the contribution of triple excitations is approximately treated in a hybrid manner [denoted as CCSD(T)-h], is presented. In this approach, canonical UHF molecular orbitals are first transformed into corresponding orbitals so that each alpha-spin orbital is paired with only one beta-spin orbital. Then, active orbitals (occupied or virtual) are automatically selected by setting a threshold for the overlap integrals of corresponding orbitals. With the concept of active orbitals, triple excitations can be divided into two subsets: (1) "active" triples involving at least one occupied active orbital and one virtual active orbital and (2) the remaining triples. The amplitudes of these two classes of triple excitations are obtained via two different approaches. When the present method is employed to study bond-breaking processes, it computationally scales as the seventh power of the system size, because the number of active orbitals involved in such processes is relatively small compared to the total number of the orbitals, and is usually independent on the system size. It has been applied to study the bond-breaking potential energy surfaces in the H(8) model and five small molecules (HF, F(2), CH(4), H(2)O, and N(2)). For all systems under study, the overall performance of CCSD(T)-h is very competitive with that of CCSDT, and much better than that of the UHF-based CCSD(T).
本文提出了一种基于非限制哈特ree-fock（UHF）基准的近似耦合簇单双三激发（CCSDT）方法，其中三激发的贡献以混合方式进行近似处理（表示为 CCSD(T)-h）。在这种方法中，首先将规范的 UHF 分子轨道转换为相应的轨道，使得每个 alpha 自旋轨道仅与一个 beta 自旋轨道配对。然后，通过设置相应轨道重叠积分的阈值，自动选择活性轨道（占据或虚拟）。利用活性轨道的概念，可以将三激发分为两个子集：（1）“活性”三激发，涉及至少一个占据的活性轨道和一个虚拟的活性轨道；（2）其余的三激发。这两类三激发的振幅通过两种不同的方法获得。当本方法用于研究键断裂过程时，其计算复杂度为系统大小的七次方，因为涉及的活性轨道数量相对于轨道总数来说相对较小，并且通常与系统大小无关。该方法已应用于研究 H(8)模型和五个小分子（HF、F(2)、CH(4)、H(2)O 和 N(2))的键断裂势能面。对于所有研究的系统，CCSD(T)-h 的整体性能与 CCSDT 非常竞争，并且比基于 UHF 的 CCSD(T)更好。
