Not at all unusual, Han Sen. If you think about the EXAFS equation, you'll see that sigma^2 and amplitude primarily affect the amplitude of the signal, while distances affect the position of the peak in the Fourier transform (or equivalently, the spacing of peaks in chi(k)). So sigma^2 and amplitude can trade off without affecting distance- based aspects of the fit much. That's why I suggested you try forcing the sigma^2 to a "reasonable" value to see what happened to your fit. Sometimes none of the aspects of the fit you're interested in depend strongly on the sigma^2 of low- amplitude paths--particularly if what you're interest in is distances or information that is in part derived from distances, like phase identification. In those cases, the anomalous sigma^2 can be a "yellow flag" (think about what might be causing it and decide if it's a problem to your scientific case) rather than a "red flag" (drop everything and resolve the problem before proceeding). Also, note from the EXAFS equation that sigma^2 is weighted by k^2, and amplitude is not. If fits using different k-weights result in significantly different values of sigma^2, that can be a clue that the issue is actually one of amplitude, as in your case. At any rate, I'm glad you solved your issue in such a satisfying way! --Scott Calvin Sarah Lawrence College On Oct 7, 2010, at 11:22 AM, Han Sen Soo wrote:

Hello Shelly and Scott, Thank you both again for your suggestions. It seems that after making the MS path more linear in my cif file, the FEFF calculation increased the amplitude value of the path and dramatically increased the sigma^2 value in the fit. Strangely, the fit values for the distances remain pretty much the same and the statistical figures of merit have improved, but the sigma^2 values are now much more reasonable (about twice as large, but I have a more triangular than linear model, so you're right Scott, your explanation does not work for my case). I guess the increased amplitude made a difference?

Hello Abhijeet, I used a rudimentary geometrical way to get my bond angles. For a 3 atom triangle M-O-A, the effective MS path length (R_MOA) is twice the sum of the individual bond distances. So if you have the R_MOA, R_MO, and R_MA distances from your fits, you can use R_MOA - R_MO - R_MA to get the O-A bond length. And with the 3 sides of the triangle, you can use the geometrical Cosine Rule to get any of the 3 bond angles. This is just geometry so I don't know what the error propagation for this would be.

Thanks again everyone! han sen