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

[Bruce Ravel]

On Monday, August 20, 2012 09:53:13 AM Wei Li wrote:
> Hi, Bruce,
> 
> Does that mean each shell has a different S02 value?

No.  It means that S02 must be evaluated for each path.  

Or, I suppose, I should say that the product of N and S02 must be
evaluated for each path.  

No!  No!  Actually, I should say that the product of N and S02 must be
evaluated correctly for each path where what I mean by "correctly"
depends upon the details of your fitting model.

That sounds right!




If you are fitting your copper foil data, then "correctly" probably
means to let N be the path degeneracy found by the path finder and to
use a single S02 parameter for all paths.


If you are fitting a crystal with different sites for the absorbing
atom -- say the super conductor YBa2Cu3O7, which has 1 Cu atom in the
Cu1 site and 2 Cu atoms in the Cu2 site -- then your parameterization
of S02 must be scaled accordingly depending on which Feff calculation
the path comes from.


If you are fitting an amorphous solid or a nanoparticle or something
else which unknown coordination, then you are probably best off
setting N to 1 and parameterizing S02 for each path in some manner
that captures the details of the coordination environment that you are
trying to measure.


Remember, N and S02 are multiplied together in the EXAFS equation.

HTH,
B


> Wei
> 
> On Mon, Aug 20, 2012 at 9:44 AM, Bruce Ravel <bravel at bnl.gov> wrote:
> > 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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-- 

 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