Hi all, I thought I'd add my two cents: I agree with everything Bruce said (!), but I think it is worth emphasizing one aspect of this issue. Basically, unless I'm using a mutiple-element fit with some clever constraints (not yet available in Artemis, but soon, I gather), any determination of coordination number is likely to be pretty uncertain. Not only does N correlate completely with S02 for a single shell fit, it is also likely to correlate strongly with sigma2 (or the Debye or Einstein temperature or whatever I'm using as the free parameter). Multiple-shell fits help when I can use them, but if I don't know the coordination number for the first shell, how likely is it that I know the structure of the material well enough to fit outer shells? Although you may see coordination numbers determined by EXAFS with an "uncertainty" of 1 in publications, this is usually something like the uncertainty reported in IFEFFIT--it probabIy doesn't include systematic errors like the effect of assumptions concerning the Debye-Waller factor or S02. In most cases if I'm looking for a first shell coordination number via EXAFS I'm not counting on better than a factor of two accuracy. That may still be useful in some cases, but doesn't help distinguish between, say, tetrahedral and octahedral coordination. So, if I'm only looking for a factor of 2, and if I'm pretty sure my sample is homogenous and of a consistent and appropriate thickness, I'm happy to just constrain S02 to 0.90 or 0.85 or whatever, knowing that the arbitrary choice of S02 is probably introducing an uncertainty of less than 20% in the coordination number. Since this is still considerably less than I generally believe is introduced by other issues (such as the scheme used for the Debye-Waller factors), it's good enough for me. --Scott Calvin Naval Research Lab --