Zhang Ning, Xiao Yunlong, Liu Wenjian
Beijing National Laboratory for Molecular Sciences, Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China.
Qingdao Institute for Theoretical and Computational Sciences, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao, Shandong 266237, People's Republic of China.
J Phys Condens Matter. 2022 Apr 1;34(22). doi: 10.1088/1361-648X/ac5db4.
The near-exact iCIPT2 approach for strongly correlated systems of electrons, which stems from the combination of iterative configuration interaction (iCI, an exact solver of full CI) with configuration selection for static correlation and second-order perturbation theory (PT2) for dynamic correlation, is extended to the relativistic domain. In the spirit of spin separation, relativistic effects are treated in two steps: scalar relativity is treated by the infinite-order, spin-free part of the exact two-component (X2C) relativistic Hamiltonian, whereas spin-orbit coupling (SOC) is treated by the first-order, Douglas-Kroll-Hess-like SOC operator derived from the same X2C Hamiltonian. Two possible combinations of iCIPT2 with SOC are considered, i.e., SOiCI and iCISO. The former treats SOC and electron correlation on an equal footing, whereas the latter treats SOC in the spirit of state interaction, by constructing and diagonalizing an effective spin-orbit Hamiltonian matrix in a small number of correlated scalar states. Both double group and time reversal symmetries are incorporated to simplify the computation. Pilot applications reveal that SOiCI is very accurate for the spin-orbit splitting (SOS) of heavy atoms, whereas the computationally very cheap iCISO can safely be applied to the SOS of light atoms and even of systems containing heavy atoms when SOC is largely quenched by ligand fields.
用于强关联电子系统的近似精确的iCIPT2方法源于迭代组态相互作用（iCI，全CI的精确求解器）与静态关联的组态选择以及动态关联的二阶微扰理论（PT2）的结合，该方法已扩展到相对论领域。本着自旋分离的精神，相对论效应分两步处理：标量相对论由精确二分量（X2C）相对论哈密顿量的无穷阶、无自旋部分处理，而自旋轨道耦合（SOC）由从同一X2C哈密顿量导出的一阶、类Douglas-Kroll-Hess的SOC算符处理。考虑了iCIPT2与SOC的两种可能组合，即SOiCI和iCISO。前者以同等方式处理SOC和电子关联，而后者本着态相互作用的精神处理SOC，通过在少数相关标量态中构造并对角化一个有效的自旋轨道哈密顿矩阵。双群和时间反演对称性都被纳入以简化计算。初步应用表明，SOiCI对于重原子的自旋轨道分裂（SOS）非常准确，而计算成本非常低的iCISO在SOC被配体场大量淬灭时，可以安全地应用于轻原子甚至包含重原子的系统的SOS。
