[Ifeffit] How to fit "N" number of Neigbors in (D)Artemis?
scalvin at sarahlawrence.edu
Mon Aug 20 09:24:42 CDT 2012
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...
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?
> 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 ------------------------------------ bravel at bnl.gov
> > National Institute of Standards and Technology
> > Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
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