Dear mailing list - This is a general EXAFS question I decided to post in regards to your collective thoughts and/or experience with this EXAFS analysis approach. I am currently attempting to model a number of Fe-S clusters with nuclearities of 2, 3, 4, 5, and 6. In some cases (where all the Fe atoms are chemically equivalent i.e. same oxidation state and coordination geometry) it is easy to approach this by assuming a single Fe absorber model. When the Fe atoms become inequivalent (oxidation states which may be a mixture of I, II or III, and different coordination geometry) I can average the environments (by including all interactions and ajusting coordination numbers for the different iron atoms accordingly) again into a single Fe absorber model. The fits go well, giving realiable bond lengths (when compared to available crystallographic data), DW's, and statistics (I am also keeping in mind the number of refined parameters to ensure a well over-determined fit for each of the models). My only hesitation, and main reason I am posting this question, is that I am not completely sure this is a reasonable assumption as the Fe atoms are in different oxidation states and will have different E0's. What I do know is there is a shift of roughly 2 to 3 eV in E0 to higher energy for every increase, by one, in oxidation state (i.e. Fe(I) to Fe(II)). Is there any way of determining if this small difference in energy, and hence E0, between the Fe atoms in the clusters can or cannot be neglected in modelling the EXAFS? Regards, Mark -- Hi Mark, I liked Bruce's response, but since you asked again. I will go out on a limb. I believe that your assumption is justified. The theoretical models built using Feff7 or less do not have different Ezero values based on the valence state of the absorbing atom. The difference is made by choosing a slightly different Ezero at the edge, rather than a difference in K=0 in the theoretical model. In addition, different Ezero's become important at low k-values. Using a kmin of 3.0 inverse angstrom or greater, I would not expect you to need different Ezero's. If you move kmin from something like 3.0 inverse angstroms to 1.5 inverse angstroms you might find that your model no longer works with one Ezero. Also, Ezero and Delr are correlated. So say you know delr then you might be able to "see" a few eV shift in Ezero, assuming that you are using a lot of low k in the data range. If you want to see the shift. Then I would get my hands on the metal oxides for each valence state. And then simultaneously fit each oxidation state together with the mixture...You might be able to pull the info out this way. -As Bruce suggested. It is unlikely that your data range and fit range will support more than one Ezero. To verify that only one Ezero is needed: Try several models with a each shell having it's own Ezero, or two Ezeros or one Ezero. Compare the reduced chi square values of the different models. If the reduced chi square value decreases by a factor of two or more then your data will supports more than one Ezero. Also check the uncertainty in the values for Ezero. If the values overlap considering the error then these Ezero-values are the same and should be only one variable instead of two. I am sure that you can find EXAFS papers of mixed Fe(II)/Fe(III) oxides such as magnetite, Fe2O3. I would guess that they are modeled with a single Ezero. HTH Shelly