Parkhill John A, Lawler Keith, Head-Gordon Martin
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Chem Phys. 2009 Feb 28;130(8):084101. doi: 10.1063/1.3086027.
A local approximation to the Schrodinger equation in a valence active space is suggested based on coupled cluster (CC) theory. Working in a pairing active space with one virtual orbital per occupied orbital, this perfect quadruples (PQ) model is defined such that electrons are strongly correlated up to "four-at-a-time" in up to two different (occupied-virtual) electron pairs. This is a truncation of the CC theory with up to quadruple substitutions (CCSDTQ) in the active space, such that the retained amplitudes in PQ are proportional to the fourth root of the number of CCSDTQ amplitudes. Despite the apparently drastic nature of the PQ truncation, in the cases examined this model is a very accurate approximation to complete active space self-consistent field. Examples include deformations of square H(4), dissociation of two single bonds (water), a double bond (ethene), and a triple bond (nitrogen). The computational scaling of the model (fourth order with molecule size) is less than integral transformation, so relatively large systems can be addressed with improved accuracy relative to earlier methods such as perfect and imperfect pairing, which are truncations of CCSD in an active space.
基于耦合簇（CC）理论，提出了一种价层活性空间中薛定谔方程的局部近似方法。在每个占据轨道有一个虚拟轨道的配对活性空间中工作，定义了这种完美四重态（PQ）模型，使得电子在多达两个不同的（占据 - 虚拟）电子对中“一次四个”地强烈关联。这是活性空间中具有多达四重替换（CCSDTQ）的CC理论的一种截断，使得PQ中保留的振幅与CCSDTQ振幅数量的四次方根成正比。尽管PQ截断的性质看似很极端，但在所研究的案例中，该模型是对完全活性空间自洽场的非常精确的近似。例子包括方形H(4)的变形、两个单键（水）、一个双键（乙烯）和一个三键（氮气）的解离。该模型的计算标度（与分子大小成四次方关系）小于积分变换，因此相对于早期方法（如完美和不完美配对，它们是活性空间中CCSD的截断），可以以更高的精度处理相对较大的系统。
