To understand the origin of the variation in the zero-field splitting and hence energy barrier to relaxation in complexes [Co(1-3)](ClO4)2, calculations using the experimental geometries from SCXRD data were used to determine the splitting of the d-orbital energy levels (Figure ). Energy levels were determined using state average CASSCF, as implemented in the Orca program, taking into account relativistic and SOC effects with an active space considering 7 electrons and 5 roots in the state averaging (SA-5-CASSCF [7,5]).