SOiCISCF: Combining SOiCI and iCISCF for Variational Treatment of Spin–Orbit Coupling
Yang Guo, Ning Zhang, Wenjian Liu
Abstract
It has recently been shown that the SOiCI approach [ Zhang, N.; J. Phys.: Condens. Matter 2022, 34, 224007 ], in conjunction with the spin-separated exact two-component relativistic Hamiltonian, can provide very accurate fine structures of systems containing heavy elements by treating electron correlation and spin–orbit coupling (SOC) on an equal footing. Nonetheless, orbital relaxations/polarizations induced by SOC are not yet fully accounted for due to the use of scalar relativistic orbitals. This issue can be resolved by further optimizing the still real-valued orbitals self-consistently in the presence of SOC, as done in the spin–orbit coupled CASSCF approach [ Ganyushin, D.; et al. J. Chem. Phys. 2013, 138, 104113 ] but with the iCISCF algorithm [ Guo, Y.; J. Chem. Theory Comput. 2021, 17, 7545–7561] for large active spaces. The resulting SOiCISCF employs both double group and time reversal symmetries for computational efficiency and the assignment of target states. The fine structures of p-block elements are taken as showcases to reveal the efficacy of SOiCISCF.