Köhn Andreas, Bargholz Arne
Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
J Chem Phys. 2019 Jul 28;151(4):041106. doi: 10.1063/1.5115045.
The second-order approximate internally contracted multireference coupled-cluster singles and doubles method icMRCC2 is defined and tested. The method is designed to bridge the gap between multireference perturbation theory and single-reference second-order approximate coupled-cluster theory (CC2). By including semi-internal double excitations into the zeroth-order expansion, the new method is able to reliably describe the coupling between excitations within the active space and the entire single-excitation spectrum. This helps, for instance, to provide a balanced treatment of valence and ionic states in polyenes (as explicitly demonstrated for cyclopentadiene) and to arrive at a more complete coverage of the excitation spectrum without the need to include diffuse orbitals into the active space. Good performance is also seen for notoriously difficult molecules such as C and CN. Furthermore, the multireference extension removes the main failures of single-reference CC2 theory, such as in the case of ozone.
定义并测试了二阶近似内收缩多参考耦合簇单双激发方法icMRCC2。该方法旨在弥合多参考微扰理论与单参考二阶近似耦合簇理论（CC2）之间的差距。通过将半内双激发纳入零阶展开，新方法能够可靠地描述活性空间内激发与整个单激发光谱之间的耦合。例如，这有助于对多烯中的价态和离子态进行平衡处理（如环戊二烯的具体示例所示），并在无需将弥散轨道纳入活性空间的情况下更全面地覆盖激发光谱。对于诸如C和CN等 notoriously difficult 分子也表现出良好性能。此外，多参考扩展消除了单参考CC2理论的主要缺陷，如在臭氧的情况下。
