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An alternative hierarchy of electron correlation beyond the electron pair approximation

机译:除了电子对近似之外,电子相关性的替代层次

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摘要

Starting from the Nakatsuji theorem, a hierarchy of approximations is considered that begins with traditional coupled cluster theory with singles and doubles (CCSD) and proceeds via the ansatz of semigeneralized singles and doubles (CCSGSD), with operators of the types ?ic_(ab) and ?ij_(ka) included, to the generalized singles and doubles (CCGSD) ansatz with the full basis {?rs_(pq); ?q_p}, and if necessary, beyond. The simplest realization of CCSGSD is the constrained coupled cluster ansatz with singles, doubles, and triples (CCCSDT). It is related to traditional coupled cluster with singles, doubles, and triples (CCSDT), but it requires (for sufficiently large n) a smaller number of parameters of O(nm~3) vs O(n3m~3) in CCSDT, n being the number of electrons and m the size of the basis. A linear system of equations for the CCCD[T] approximation is derived and strategies toward its solution are discussed.
机译:从Nakatsuji定理开始,考虑了近似的层次结构,该层次结构从具有单打和双打(CCSD)的传统耦合聚类理论开始,并通过半广义单打和双打(CCSGSD)的anstz进行,运算符类型为?ic_(ab)和?ij_(ka)包含到具有完整基础的广义单打和双打(CCGSD)ansatz {?rs_(pq); ?q_p},必要时还可以。 CCSGSD的最简单实现是具有单,双和三重(CCCSDT)的约束耦合簇ansatz。它与具有单,双和三元组(CCSDT)的传统耦合群集有关,但是(对于足够大的n),需要较少数量的O(nm〜3)与CCSDT中的O(n3m〜3)参数,n是电子的数量和m的基数。推导了用于CCCD [T]近似的线性方程组,并讨论了其求解方法。

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