Assessment of DFT Functionals for Structural Determination of Lanthanide(III) Complexes Using Ligand Field Splitting

Lanthanide (Ln³⁺) tetrakis complexes, C[Ln(L)₄], are important for applications due to their high quantum yields, solubility, and stability. Their luminescent properties depend on the structure, particularly the coordination polyhedron, the assessment of computational methods for calculating their structures is paramount. Usually, this assessment uses the RMSD of distances in the [Ln(L)₄]⁻ complex or {LnO₈} polyhedron between crystallographic and calculated structures. However, since ligand field (LF) splitting is highly geometry-dependent, the RMSD between experimental LF splitting and Stark levels (RMSD-LF) offers a more accurate measure for evaluating quantum methods. LF energy eigenvalues were calculated using the simple overlap model (SOM), with geometries optimized by various density functionals. M06 and M06-L functionals, with def2-SVP/MWB52(Eu)/CPCM, demonstrate the best balance in best accuracy and low computational cost, making them suitable for modeling C[Eu(L)₄] complexes.