Aoto Yuri Alexandre, Köhn Andreas
Institut für Theoretische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
J Chem Phys. 2016 Feb 21;144(7):074103. doi: 10.1063/1.4941604.
The internally contracted multireference coupled cluster (icMRCC) theory is reexamined in a multistate framework. The new derivation starts from the Bloch equations employing a wave operator similar to the one originally employed by Jeziorski and Monkhorst [Phys. Rev. A 24, 1668 (1981)], but allows for a multideterminantal model-space basis. The resulting working equations are a generalization of state-universal multireference coupled-cluster (SU-MRCC) theory. In the case of truncated cluster operators, energies and wave functions depend on the choice of the model-space basis. It is shown that the state-specific solutions of the original icMRCC theory exactly diagonalize the effective Hamiltonian of the multistate theory and thus decouple the multistate equations. While this in principle means that icMRCC is a multistate theory, we find that truncated icMRCC theory still shows minor artifacts close to avoided crossings, which can be removed by considering a true multistate ansatz. The results also shed new light on the orbital variance of the original SU-MRCC theory.
在多态框架下重新审视了内收缩多参考耦合簇（icMRCC）理论。新的推导从布洛赫方程开始，采用了一个类似于耶齐尔斯基和蒙克霍斯特最初使用的波算子[《物理评论A》24, 1668 (1981)]，但允许使用多行列式模型空间基。由此产生的工作方程是状态通用多参考耦合簇（SU-MRCC）理论的推广。在截断簇算子的情况下，能量和波函数取决于模型空间基的选择。结果表明，原始icMRCC理论的状态特定解能精确对角化多态理论的有效哈密顿量，从而使多态方程解耦。虽然这原则上意味着icMRCC是一种多态理论，但我们发现截断的icMRCC理论在接近避免交叉处仍显示出微小的伪影，通过考虑真正的多态假设可以消除这些伪影。这些结果也为原始SU-MRCC理论的轨道方差提供了新的见解。
