Unexpectedly large basis set effects on the binding of O₂ to heme complexes

IMOMM(Becke3LYP:MM3) geometry optimizations on the Fe(P)(H2O)(O2) (P = porphyrin) system with different basis sets are shown to provide large differences in the iron-oxygen bond distance. This difference is found to be not in the basis set superposition error or in the description of the interaction between iron and dioxygen. Instead, the origin of the difference is shown to be in the relative energies of the FeII(P)(O2) and FeIII(P)(O2-) descriptions emerging from the stabilities of the separate FeII/O2 and FeIII/O2- fragments.