Density functional theory optimizations have been conducted on structures of complexes of Fe2+ With (H2O)(n) (n = 0-3) in three-residue models of binding sites A and B of the ferroxidase center of bullfrog M ferritin. Each site is modeled by the full structures of its three active amino acids. The potential surface at each site in the presence of water molecules is complex; coordination numbers of iron from three to six are seen. Water contributes to the complexity through its ability to hydrogen bond, to coordinate to iron, and to displace the neutral ligands glutamine and histidine. Intrinsic differences are noted at each site; at site B, the most stable complexes are found to favor tetracoordinate iron, while pentacoordination is preferred at site A in the two- and three-water complexes. While each incremental addition of a water molecule results in increased stability, successive binding energies are found to decrease.