[Ifeffit] How to fit "N" number of Neigbors in (D)Artemis?

[Scott Calvin]

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  ------------------------------------ bravel at 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 
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