,60 and also by Hermitian quasi-degenerate NEVPT2 variant (QD-NEVPT2). All calculations were done with ORCA 5.0 software. The molecular structures were derived from the experimental X-ray data, and only the atomic positions of hydrogen were normalized with Mercury software. The ZORA relativistic approximation was used, together with ZORA-def2-TZVPP for iron atoms, ZORA-def2-TZVP(-f) for lighter atoms, and ZORA-def2-SVP for C and H atoms. The calculations were sped up using the SARC/J Coulomb fitting basis set and the RIJCOSX approximation. The largest integration grid (DEFGrid3) and tightSCF convergence criteria were used in all calculations. First, the results for crystal structure measured at 100 K are discussed. Usually, the active space in metal complexes is based on their d-orbitals and such calculations are summarized in Figure . With the help of ab initio ligand field theory (AILFT) the energies of d-orbitals were calculated, and their energies are ordered as dxy < dxz, dyz < dz2 < dx2-y2 as expected for square-pyramidal arrangement (Figure ). However, the lowest quartet ligand field term is located at 9396 cm -1 (QD-NEVPT2), which is far away from the -value derived from magnetic data. High energy separation of the excited states also caused a small value of the axial ZFS parameter, D = 2.1 cm -1 . This inspired further calculations with large active spaces; first, the -type ligand-based orbital was added, forming CAS(7e,6o). Next, the more diffuse 4d metal orbitals were added resulting in CAS(7e,11o).