The CASSCF/CASPT2 calculations were performed using OpenMolcas 18.09. The active space (10e/12MO) was constructed from the following orbitals to account for both MC and MLCT states: three non-bonding 3d-orbitals of the iron atom as well as three corresponding Rydberg-like 4d orbitals to include the double-shell effect, two σ d -bonding and two σ * d -antibonding orbitals, and the two vacant π * -orbitals of the ligands (see Figure )). The highest possible Abelian point symmetry group C 2 , Cholesky decomposition of the twoelectron integral matrix and extended relativistic ANO-RCC-TZVP basis set together with Douglas-Kroll-Hess approach for scalar relativistic effects or 6-31G(d) were utilized. J * for S 0 J 0 for S 0 J 1 for S 0 J 0 for T MC J 0 for T MLCT J 0 for Q MC In addition, we have computed the spin-orbit coupling (SOC) matrix elements for the lowest five singlet and triplet states on a 2D (α, ω) grid. The 2-norms of the respective SOC matrix, ||H SOC || 2 , are shown in Fig. . In general, one can see a systematic increase of SOC with the amount of exact exchange, that is along the diagonal in the (α, ω) plane. In the range of parameters considered here the magnitude of ||H SOC || 2 changes by a factor of about 2.5 from 1600 to 3700 cm -1 . To estimate the influence of the (α, ω) parameters on the state splittings we also computed the 2-norms of the couplings weighted with the energy differences between zero-order states: