DFT calculations on the spin-crossover complex Fe(salen)(NO): a quest for the best functional

DFT calculations on the spin-crossover complex Fe(salen)(NO) provide a striking illustration of the comparative performance of different exchange-correlation functionals vis-à-vis the issue of transition metal spin state energetics. Thus, although the "classic" pure functionals PW91 and BLYP favor the S = 1/2 state by about 10 kcal/mol, relative to the S = 3/2 state, the hybrid functional B3LYP favors the latter state by nearly the same margin. In contrast, the newer pure functionals OLYP and OPBE, based on the OPTX exchange functional, as well as the B3LYP* hybrid functional (which has 15% Hartree-Fock exchange, compared with 20% for B3LYP) predict nearly isoenergetic S = 1/2 and 3/2 states, as required for a spin-crossover complex. Intriguingly, the OLYP and B3LYP* spin density profiles for the S = 1/2 state of Fe(salen)(NO) are substantially dissimilar.