The SOC matrix was calculated by complete active space self consistent field (CASSCF) /CASPT2 calculations. We used an active space consisting of 10 electrons correlated on the following 16 orbitals: three 3d-t 2g (d xy , d xz , d yz ) and two 3d-e * g (d x 2 -y 2 , d z 2 ) type Febased orbitals, an additional set of three 4d-t 2g and 4d-e * g orbitals, two Fe-N σ-bonding 3d-e g orbitals, and four dominantly ligand-based (terpy-π * ) orbitals that are required to access the MLCT states. We used atomic natural orbital relativistic correlation consistent (ANO-RCC) basis sets with the following contractions: (7s6p5d4f 3g2h) for Fe, (4s3p1d) for N, (3s2p) for C, and (2s) for H atoms. In the CASPT2 computations, we employed an imaginary level shift of 0.2 a.u. and a standard IPEA shift of 0.25 a.u. For CASPT2, we froze the core orbitals; in addition for CASSCF, we also froze the semi-core Fe-3s and Fe-3p z orbitals, which was required to construct the 10e16o active space.