The CASSCF/CASPT2 calculations were performed at the CASPT2(10e,12o) groundstate ( 1 GS) minimum resulting from a 2D PES scan along the Fe-N ax bond length and the NNN angle. We utilised C 2 point group symmetry with the symmetry axis z defined by the Fe-N ax bonds. All CASSCF calculations were carried out with state averaging (equal weights) over 20 states (for each spin multiplicity singlet/triplet/quintet and C 2 state symmetry A/B) with the exception of triplet A, for which we had to use 30 roots to maintain the active space. The CASPT2 calculations were carried out in multistate (MS-CASPT2) mode using 10 states for each spin multiplicity/C 2 state symmetry. The SOC matrix elements were calculated by a spin-orbit state interaction (SO-SI) method utilising MS-CASPT2 energies, CASSCF wave functions, and a one-electron effective mean-field SOC Hamiltonian. Scalar relativistic effects were taken into account using the Douglas-Kroll-Hess (DKH) Hamiltonian. All CASSCF/CASPT2 calculations were carried out using the OpenMolcas20.10 46,47 software.