General Spin Restricted Open-Shell Configuration Interaction Singles (GS-ROCIS): Implementation of Spin-Orbit Coupling and Zeeman Operators for Calculation of Optical and X-ray Absorption and Magnetic Circular Dichroism Spectra of Magnetically Coupled Transition Metal Systems.

In this paper we present the theory and implementation of the spin-orbit coupling and Zeeman operators in the context of quasi-degenerate perturbation theory into the general spin restricted open-shell configuration interaction singles method. The implementation of the mentioned operators allows for the calculation of magnetic circularly polarized dichroism (MCD), L-edge X-ray absorption spectra (XAS) and X-ray magnetic circularly polarized dichroism (XMCD) spectra. The method was tested on calculating the MCD spectra of isostructural complexes [LCr^III(PyA)₃Ni^II]²⁺, [LCr^III(PyA)₃Zn^II]²⁺and [LGa^III(PyA)₃Ni^II]²⁺, with L = 1,4,7-trimethyl-1,4,7-triazacyclonanane and PyA^- is the monoanion of pyridine-2-aldozime, where it correctly predicts the MCD signs of the lower optical transition of [LCr^III(PyA)₃Ni^II]²⁺and [LGa^III(PyA)₃Ni^II]²⁺. The capabilities of the method in computing L-edge XAS and XMCD spectra were tested on the model complexes [Cu(H₂O)₆]²⁺ and [Cu₂(OAc)₄(H₂O)₂], where it correctly calculates the L_2,3-edge absorption and XMCD spectra, as well as on the antiferromagnetically coupled Cu-Fe dimer [(F₈TPP)Fe(μ-O)Cu(TMPA)]⁺, where it correctly predicts the signs of the L₂ and L₃ edges of the Cu XMCD spectrum. To further illustrate the applicability of the method, the more complex L_2,3-edge XAS and XMCD spectra of thiolate Fe complexes were also calculated.