Abstract
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)(3)Ni(II)](2+), [LCr(III)(PyA)(3)Zn(II)](2+)and [LGa(III)(PyA)(3)Ni(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)(3)Ni(II)](2+)and [LGa(III)(PyA)(3)Ni(II)](2+). The capabilities of the method in computing L-edge XAS and XMCD spectra were tested on the model complexes [Cu(H(2)O)(6)](2+) and [Cu(2)(OAc)(4)(H(2)O)(2)], where it correctly calculates the L(2,3)-edge absorption and XMCD spectra, as well as on the antiferromagnetically coupled Cu-Fe dimer [(F(8)TPP)Fe(μ-O)Cu(TMPA)](+), where it correctly predicts the signs of the L(2) and L(3) 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.