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

IMOMM(Becke3LYP:MM3) geometry optimizations on the Fe(P)(H2O)(O-2) (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 Fe-II(P)(O-2) and Fe-III(P)(O-2(-)) descriptions emerging from the stabilities of the separate (FeO2)-O-II and Fe-III/O-2(-) fragments. (C) 2001 John Wiley & Sons, Inc.