Chakravarti Dibyajyoti, Sen Sangita, Mukherjee Debashis
Centre for Quantum Engineering, Research, and Education (CQuERE), TCG CREST, Kolkata, India.
Department of Chemical Sciences, Indian Institute of Science, Education and Research, Kolkata, India.
J Chem Phys. 2023 Oct 7;159(13). doi: 10.1063/5.0168941.
The Unitary Group Adaptation (UGA) offers a very compact and efficient spin adaptation strategy for any spin-free Hamiltonian in a many body framework. Our use of UGA in the context of state-specific (SS) Jeziorski-Monkhorst Ansatz based multireference coupled cluster (MRCC) theory obviates the non-commutativity between the spin-free cluster operators via a normal ordered exponential parametrization in the wave operator. A previous formulation of UGA-SSMRCC by us [R. Maitra, D. Sinha, and D. Mukherjee, J. Chem. Phys. 137, 024105 (2012)], using the same ansatz, employed certain sufficiency conditions to reach the final working equations, which cannot be improved systematically. In this article, we will present a more rigorous formulation that follows from an exact factorization of the unlinked terms of the Bloch equation, resulting in equations on which a hierarchy of approximations can be systematically performed on the emergent additional terms. This derivation was shown in our recent article [D. Chakravarti, S. Sen, and D. Mukherjee, Mol. Phys. 119, e1979676 (2021)] in the context of a single open shell CC formalism and was applied to spectroscopic energy differences where the contribution of the new terms was found to be of the order of ∼0.001 eV for ionization potential, electron affinity, and excitation energy. In the current work, we will present a comparison between the earlier and current formulations via both a theoretical analysis and a numerical demonstration of the dramatic effect of the additional terms brought in by the factorization on potential energy curves. The contribution of such terms was found to gain importance with an increase in the number of singly occupied active orbitals in the model space functions.
酉群适配（UGA）为多体框架下的任何无自旋哈密顿量提供了一种非常紧凑且高效的自旋适配策略。我们在基于特定态（SS）耶齐尔斯基 - 蒙克霍斯特假设的多参考耦合簇（MRCC）理论中使用UGA，通过波算符中的正规序指数参数化消除了无自旋簇算符之间的非对易性。我们之前[R. 迈特拉、D. 辛哈和D. 慕克吉，《化学物理杂志》137, 024105 (2012)]使用相同假设对UGA - SSMRCC的一种表述，采用了某些充分条件来得到最终的工作方程，而这些条件无法系统地改进。在本文中，我们将给出一种更严格的表述，它源于布洛赫方程非关联项的精确因式分解，从而得到一些方程，在这些方程上可以对出现的附加项系统地进行一系列近似。这种推导在我们最近的文章[D. 恰克拉瓦蒂、S. 森和D. 慕克吉，《分子物理》119, e1979676 (2021)]中在单开壳层CC形式体系的背景下给出，并应用于光谱能量差，其中发现新项对电离势、电子亲和能和激发能的贡献约为0.001 eV量级。在当前工作中，我们将通过理论分析和数值演示，对早期和当前的表述进行比较，展示因式分解引入的附加项对势能曲线的显著影响。发现随着模型空间函数中单占据活性轨道数量的增加，这些项的贡献变得更加重要。
