Unexpectedly large basis set effects on the binding of O-2 to heme complexes

IMOMM(Becke3LYP:MM3) geometry optimizations on the Fe(P)(H2O)(O-2) (P = por phyrin) system with different basis sets are shown to provide large differe nces in the iron-oxygen bond distance. This difference is found to be not i n the basis set superposition error or in the description of the interactio n 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.