Just to fill out some details about how this somewhat quirky aspect of Ifeffit/Artemis can be used:
Suppose you want to find the degeneracy of nearest-neighbors, represented by path 1, and next-nearest neighbors, represented by path 2. To fit the degeneracy of one of the paths, 1. Set "N" to 1 for both paths. 2. Create parameters named, say, "amp," "N1," and "N2." 3. Fill in the N box for both the first and second path with "1" (not "N1" and "N2"!) 4. Fill in the S02 box for path 1 with "amp*N1" and for path 2 with "amp*N2." 5. EXAFS does not contain enough information to guess all three parameters; one of them must be set. Often, that is amp, perhaps because it is obtained from a standard. But maybe it's N1: you might know the near-neighbor coordination, but not the next-nearest neighbors. In any case, you need to set at least one of the three parameters to a known (or suspected) value. 6. All your named parameters now mean what you think they should mean in the log files: e.g., "N2" is the coordination number for the next-nearest neighbors. But the "S02" path parameter in the log files is amp*N1 for the first path and amp*N2 for the second path. That's kind of confusing, but because you've designed your guess/def/set parameters effectively, it's not a problem! Hope that helps... --Scott Calvin Sarah Lawrence College On Aug 20, 2012, at 9:24 AM, Huaming Guo wrote: > > Thanks. In this case, is the number of neibors equal to N * SO2 in the model > result? > > Regards > > HM > > Bruce Ravel 写: > > > On Monday, August 20, 2012 09:02:28 PM Huaming Guo wrote: > >> I am using the (D)Artemis for EXAFS fitting. However, the "N" is set to be > >> a > >> constant in the (D)Artemis. We can not set the variable for this > >> parameter. > >> How can I do that if I model non crystalline samples using (D)Artemis? > >> Thanks a lot. > > > > In Artemis, N is a pure number, but S02 can take a math expression. > > > > In the EXAFS equation, N and S02 are multiplied together. > > > > Thus, you might consider parameterizing the amplitude by setting N to > > 1 and putting all of the parameterization in the math expression for > > S02. > > > > B > > > > > > -- > > > > Bruce Ravel ------------------------------------ bra...@bnl.gov > > > > National Institute of Standards and Technology > > Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 > > Building 535A > > Upton NY, 11973 > > > > Homepage: http://xafs.org/BruceRavel > > Software: https://github.com/bruceravel > > _______________________________________________ > > 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
