Bachhar Arnab, Mayhall Nicholas J
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24060, United States.
Virginia Tech Center for Quantum Information Science and Engineering, Blacksburg, Virginia 24061, United States.
J Phys Chem A. 2024 Oct 17;128(41):9015-9027. doi: 10.1021/acs.jpca.4c03914. Epub 2024 Oct 7.
The cluster-based Mean Field method (cMF) and it is second order perturbative correction was introduced by Jiménez-Hoyos and Scuseria to reduce the cost of modeling strongly correlated systems by dividing an active space up into small clusters, which are individually solved in the mean-field presence of each other. In that work, clusters with unpaired electrons are treated by allowing the α and β orbitals to spin polarize. While that provided significant energetic stabilization, the resulting cMF wave function was spin-contaminated, making it difficult to use as a reference state for spin-pure post-cMF methods. In this work, we propose the Restricted Open-shell cMF (RO-cMF) method, extending the cMF approach to systems with open-shell clusters, while not permitting spin-polarization. While the resulting RO-cMF energies are necessarily higher in energy than the unrestricted orbital cMF, the new RO-cMF provides a simple reference state for post-cMF methods that recover the missing intercluster correlations. We provide a detailed explanation of the method, and report demonstrative calculations of exchange coupling constants for three systems: a di-iron complex, a dichromium complex, and a dimerized organic radical. We also report the first perturbatively corrected RO-cMF-PT2 results as well.
基于团簇的平均场方法（cMF）及其二阶微扰校正由希门尼斯 - 霍约斯和斯库塞里亚提出，通过将活性空间划分为小团簇来降低对强关联系统建模的成本，这些小团簇在彼此的平均场存在下单独求解。在该工作中，通过允许α和β轨道自旋极化来处理具有未配对电子的团簇。虽然这提供了显著的能量稳定化，但所得的cMF波函数存在自旋污染，使其难以用作自旋纯的后cMF方法的参考态。在这项工作中，我们提出了受限开壳层cMF（RO - cMF）方法，将cMF方法扩展到具有开壳层团簇的系统，同时不允许自旋极化。虽然所得的RO - cMF能量必然高于无限制轨道cMF的能量，但新的RO - cMF为恢复缺失的团簇间相关性的后cMF方法提供了一个简单的参考态。我们对该方法进行了详细解释，并报告了三个系统的交换耦合常数的示范计算：一个二铁配合物、一个二铬配合物和一个二聚化有机自由基。我们还报告了首个微扰校正的RO - cMF - PT2结果。
