The single-reference coupled-cluster method has proven very effective in the ab initio description of atomic and molecular systems, but its successful application is limited to states dominated by a single Slater determinant, which is used as the reference. In cases where several determinants are important in the wave function expansion, i.e., we have to deal with nondynamic correlation effects, a multi-reference version of the coupled-cluster method is required. The multi-reference coupled-cluster approaches are based on the effective Hamiltonian formulation providing a two-step procedure, in which dynamic correlation effects can be efficiently evaluated by the wave operator, while nondynamic correlation contributions are given by diagonalization of the effective Hamiltonian in the final step. There are two classical multi-reference coupled-cluster formulations. In this paper, the focus is on the so-called Fock-space coupled-cluster method in its basic version with one- and two-particle operators in the exponent. Computational schemes using this truncation of the cluster operator have been successfully applied in calculations in one- and two-valence sectors of the Fock space. In this paper, we show that the approach can be easily extended and effectively employed in the three-valence sector calculations.

中文翻译：

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将Fock-space耦合群集方法从单价和双价扩展到三价扇区。

从头开始，单参考耦合簇方法已被证明非常有效原子和分子系统的描述，但其成功应用仅限于以单个Slater行列式为主导的状态，该状态用作参考。在波动函数扩展中有几个行列式很重要的情况下，即我们必须处理非动态相关效应，需要使用多参考版本的耦合聚类方法。多参考耦合聚类方法基于有效的哈密顿量公式，提供了两步过程，其中波相关算子可以有效地评估动态相关效果，而非动态相关贡献则是通过将有效哈密顿量对角线化而得到的。最后一步。有两种经典的多参考耦合集群公式。在本文中，重点是基本形式的所谓Fock空间耦合聚类方法，该方法的指数为一粒子和二粒子运算符。使用聚类运算符的这种截断的计算方案已成功地应用于Fock空间的一价和二价扇区的计算中。在本文中，我们表明该方法可以轻松扩展并有效地应用于三价扇形计算。