Hi Yanyun, This topic has been dealt with multiple times on the mailing list. Please search the archive:
http://www.mail-archive.com/ifeffit@millenia.cars.aps.anl.gov/ For example, searching for "sites" will give you several discussions of the topic. --Scott Calvin Sarah Lawrence College On Aug 19, 2014, at 2:49 PM, <huyan...@physics.utoronto.ca> <huyan...@physics.utoronto.ca> wrote: > Hi all, > > How would you deal with the case that the absorber atoms are occupying > two different crystallography sites? Should I still use/guess the same > 'S02' for paths calculated from two sites? > > Best, > Yanyun > > > Quoting huyan...@physics.utoronto.ca: > >> Hi Scott, >> >> Thank you for giving me detailed examples. I will go with your >> suggestion and try to reduce free parameters. >> >> Best, >> Yanyun >> Quoting Scott Calvin <scal...@sarahlawrence.edu>: >> >>> Hi Yanyun, >>> >>> On Aug 19, 2014, at 12:05 PM, huyan...@physics.utoronto.ca wrote: >>>> >>>> But my question is, for each path there are a set of Path Parameters, >>>> does that mean we have to fit this set of path parameters independent >>>> from those used in other paths? For instance, if I am going to >>>> include 30 paths in my fitting, do I need to guess about 150 >>>> (30*5=150) parameters (except guess the same 'enot' for all paths)? >>> >>> No. One of the great principles of Ifeffit (and thus Artemis) is >>> that the path parameters do not have to be the same as the fit >>> parameters. >>> >>> As a simple example, it's good to start with the model that all >>> paths have the same value for E0. So you can guess a single >>> parameter and then use that parameter for the E0 for all paths. >>> >>> As another simple example, a cubic crystal might be modelled as >>> having a uniform thermal (or Vegard's law) expansion. Then there >>> could be a single guessed parameter indicating the fractional >>> expansion (called, for example, alpha) while the delr for each path >>> could then be entered as alpha*reff. >>> >>> Much more complicated constraint schemes are possible, but for fits >>> with large number of paths, the number of guessed parameters is >>> almost always much much lower than the number of path parameters. >>> >>>> >>>> As shown on the fit Log file, the Correlation value between two fit >>>> parameters is the bigger the better, or the smaller the better? >>> >>> Not necessarily. Correlations are provided to help understand the >>> relationship between guessed parameters, but there's nothing >>> inherently wrong with a high correlation...the uncertainty >>> associated with that relationship is already represented in the >>> uncertainties reported with the fit. In other words, it is more >>> direct to focus on getting the uncertainties to be lower, rather >>> than the correlations. >>> >>> --Scott Calvin >>> Sarah Lawrence College >>> >>> >>> _______________________________________________ >>> Ifeffit mailing list >>> Ifeffit@millenia.cars.aps.anl.gov >>> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> >> >> >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
