Accurate Modeling of Spin-State Energetics in Spin-Crossover Systems with Modern Density Functional Theory

The energies of different spin multiplicities of a range of iron complexes are computed using modern density functional theory (DFT) methods of the generalized gradient approximation (GGA; BP86 and OPBE), meta-GGA (TPSS), hybrid meta-GGA (TPSSh), hybrid (B3LYP), and double-hybrid (B2PLYP) types. It is shown that so far only the double-hybrid density functional B2PLYP, in conjunction with large and flexible basis sets (def2-QZVPP), is able to provide qualitatively correct results of spin-state energetics for the investigated non-spin-crossover complexes. An energy difference of -6 to 0 kcal/mol is proposed to be indicative of spin-crossover behavior.