Assessing the zero‐field splitting in magnetic molecules by wave function‐based methods

The effect of spin‐orbit coupling is assessed through the spin‐orbit state interaction method. This method involves a two‐step procedure to include both dynamical electron correlation effects and spin‐orbit effects on the relative energies of the low‐lying electronic states. Accurate energies of the spin free states are obtained with the complete active space self‐consistent field/complete active space second‐order perturbation theory (CASSCF/CASPT2) procedure. Second, the spin‐orbit coupling is taken into account by interaction of the CASSCF states using the CASPT2 relative energies. The method is validated on the Co2+ ion in vacuum and in the ionic insulator CoO. Afterward the method is applied to CoCl2(PPh3)2 and the dinuclear complexes [TM2Cl6]2− (TM = Mn, Fe, Co). For the monomeric species, the results compare favorable with experimental data or fully relativistic benchmarks. We discuss the difficulties to interpret the dimer results in terms of the parameters that are usually derived from experiment.