单-双耦合簇方法的四阶微扰推广
Fourth-order perturbative extension of the singles-doubles coupled-cluster method
摘要：
导出了单价电子原子矩阵元的四阶多体修正。所得到的图进行分类，使用耦合集群启发的分离成从最低阶波函数的n粒子激发的贡献。完整的四阶图只涉及连接的单、双、三重激励和断开的四重激励。大约一半的四阶图不占流行的耦合集群方法截断在单和双激发（CCSD）。对于CCSD方法及其线性化版本遗漏的整个四阶图集，即来自连接的三重和断开的四重激励的贡献，列出了显式公式。导出的四阶贡献的部分求和方案的所有订单的微扰理论的建议。
英文摘要：
Fourth-order many-body corrections to matrix elements for atoms with one valence electron are derived. The obtained diagrams are classified using coupled-cluster-inspired separation into contributions from n-particle excitations from the lowest-order wavefunction. The complete set of fourth-order diagrams involves only connected single, double, and triple excitations and disconnected quadruple excitations. Approximately half of the fourth-order diagrams are not accounted for by the popular coupled-cluster method truncated at single and double excitations (CCSD). Explicit formulae are tabulated for the entire set of fourth-order diagrams missed by the CCSD method and its linearized version, i.e. contributions from connected triple and disconnected quadruple excitations. A partial summation scheme of the derived fourth-order contributions to all orders of perturbation theory is proposed.
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