On 08/23/2018 10:55 AM, Thomas, Andrew (AGW) wrote:
Hello,
Does anyone have any suggestions for breaking the correlation between the coordination number and sigma^2? Specifically, is there a way to set a maximum or minimum value for parameters in a fit?
Matt is, of course, correct when he says that this correlation is inherent to EXAFS analysis and cannot be "broken". I would like to suggest a broader way of thinking about this sort of issue. When you have a very well behaved analysis problem -- that is, something like my teaching example of FeS2, something for which the structure is pretty well known -- then your EXAFS analysis will yield defensible values for CN and sigma^2 without too much effort. They will still be correlated, but sensible numbers will tend to just fall out. In a harder problem -- y'know something you are doing actual research on -- you often run into the situation where these correlations preclude a "fall right out" analysis. It is tempting to assert that the CN must be SOMETHING and the ss must be SOMETHING. That's true in a sense, but you have made a real measurement and are doing a real analysis with real measurement uncertainties. And you have to respect that. When you are running into trouble in your EXAFS analysis -- big uncertainties, indefensible values, that sort of thing -- that is usually the program trying to tell you something about your analysis. Usually, that would mean that you want to know something that the data do not support (or do not support beyond some level of precision or accuracy). Or it might mean that your fitting model is not realized in the data -- that is, your model is missing some important feature and the bad fit results are the result of missing that important feature. To summarize, I want to encourage you not to assert that you need to find a way to break the correlations. i want to encourage you think about what the wonky fit results are trying to tell you about your data or your fitting model. That was kind of rambling, I admit. Hopefully it was helpful nonetheless.... B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Lead Beamline Scientist, 06BM (BMM) Building 743, Room 114 Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Beamline: https://www.bnl.gov/ps/beamlines/beamline.php?r=6-BM Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/