Shaofeng Wang asked this question on the mailing list that some may not have seen because it got marked as spam somewhere along the line. Kinda weird. Not quite sure why that happened....
I have a question about sigma square. Must this parameter be in the range of 0.003-0.02? I obtained some of it in the range of 0.0001-0.002. Is this reasonable? Could please provide some reference?
There are no hard rules for anything. That sigma^2 is 0.00something is a good guideline and often the case, but not an actual rule. The problem with a sigma^2 in the range of 0.0something is that it tends to attenuate the path so much that it doesn't really have an impact on the quality of the fit. In that case the uncertainty is often very large precisely because the path no longer contributes significant spectral weight to the fit. A sigma^2 in the range of 0.000something means that the bond is very stiff. In some situations, that would be correct. As an example, consider the sort of negative thermal expansion material that is a network of metal-ligand octahedra. The NTE effect happens in that case because, as the material heats, up the octahedra tilt within the network, pulling the network inwards. This happens because the metalligand (usually metal-oxygen) bond is very strong -- much stronger than the forces connecting the octahedra. In an EXAFS measurement of an NTE material, you will likely find a very small sigma^2 for the first shell. (And a very large one for the second shell.) Regardless of the size of sigma^2, the bottom line is that the value is defensible. This means that you can understand and explain why sigma^2 is the size that it is. It also means that the uncertainty is such that you can support your conclusion. While the red line might overplot the blue with sigma^2=0.0003+0.001, that may be a troubling result because sigma^2 is not positive definite! Executive summary: a reasonable value is a defensible value. If you are willing to explain a result to your thesis committee or let one of us review it in a manuscript, then the result is probably reasonable. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/