Assessing the performance of density functional theory for the electronic structure of metal-salens:: The d2-metals

The performance of three common combinations of density functional theory has been evaluated for the geometries and relative energies of a commonly-employed model complex of the salen ligand [salen = bis(salicylaldehydo)ethylenediamine] with the d(2)-metals Ti(II), V(III), Cr(IV), Zr(II), Nb(III), and Mo(IV). High-level ab initio methods including complete active-space third-order perturbation theory have been employed both as benchmarks for the density functional theory results and to examine the multireference character of the low-lying electronic states in these systems. The strong multireference character of the systems has been clearly demonstrated. All of the functionals examined provide geometries that are typically within 0.2 angstrom least root mean square deviation from the benchmark geometries. The performance of the density functionals for the relative energies of the low-lying electronic states is significantly worse, providing qualitatively different descriptions in some instances. Of the systems explored, no significant difference is observed in the multireference character or in the reliability of the density functional results when comparing 3d vs 4d transition-metal systems.