A PM3 parameter set for iron has been developed, which is appropriate for the active site of iron sulfur proteins having a single iron atom by fitting DFT data obtained for a redox site analogue. These parameters are then tested on a set of such analogues involving a variety of ligands. and show good agreement with both DFT and experimental data for these species. The use of these parameters within a two level ONIOM treatment of the protein rubredoxin, yields accurate predictions of the effect of the enzyme oil both Fe-S bond lengths and inner sphere reorganization energies. (C) 2005 Elsevier B.V. All rights reserved.