On Friday 02 October 2009 01:15:25 pm Hashem Stietiya wrote:
So I should not be skeptical if my fitting results return a high Debye Waller value for my first shell as compared with the second shell.
Hashem, Carlo is of course correct to preach constant scepticism. That said, there are reasons that a first shell sigma^2 might be bigger than the second shell. I don't know what they might be in your case becasue I have no idea what you are working on. Your question, though, touches upon a more general statement that is just as true when your fitting parameters are unsurprising as when they are surprising. If you cannot understand the physical meaning of your fitting parameters, if you cannot explain their values in terms of something known or suspected about the structure, then you haven't really done anything by fitting other than to make a pretty picture. *Every* parameter has to be interpretable. If there is a physical reason to expect the first shell sigma^2 to be big, fine. If you cannot understand that result in some sensible manner, then your fitting model is problematic. In any case -- surprising results or otherwise -- it is incumbant upon the person pushing the big, green button to understand and interpret the results of the fit. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/