Multireference Ab Initio Investigation on Ground and Low-Lying Excited States: Systematic Evaluation of J–J Mixing in a Eu(3+) Luminescent Complex

[Image: see text] A theoretical protocol combining density functional theory (DFT) and multireference (CAS) calculations is proposed for a Eu(3+) complex. In the complex, electronic levels of the central Eu(3+) ion are correctly calculated at the CASPT2 level of theory, and the effect of introducing different numbers of states in the configuration interaction matrices is highlighted as well as the shortcomings of DFT methods in the treatment of systems with high spin multiplicity and strong spin–orbit coupling effects. For the (5)D(0) state energy calculation, the inclusion of states with different multiplicity and the number of states considered for each multiplicity are crucial parameters, even if their relative weight is different. Indeed, the addition of triplet and singlets is important, while the number of states is relevant only for the quintets. The herein proposed protocol enables a rigorous, full ab initio treatment of Eu(3+) complex, which can be easily extended to other Ln(3+) ions.