[Ifeffit] Another stab at the data

[Matt Newville]

Dave,

On Wed, 29 Oct 2003, Dave Baker wrote:

> Bruce,
> Thanks for the advice.  I've run through the list, redone processing and
> started the analysis with said list in mind, and the most plausible
> solution is that I shouldn't be using a crystalline material for the
> FEFF calculation. One interesting thing (at least to my untrained eye)
> to note is that the shapes of the first path and the peak that I'm
> trying to fit (this is before any attempt at fitting) are nearly
> identical, but the FEFF path is shifted about 2~3 angstroms higher than
> the peak.  The only improvement that I could muster is that the e0
> variable has gone down from 80 to 50.
> -Dave Baker  

E0 shifts greater than 10eV often mean that the fit is causing the
model to "jump a period".  This often means that the backscattering
atom is incorrect or that the distance for the Feff calculation is
way, way off.   Peak shapes of |chi(R)| are difficult to interpret.  
But being 2 or 3 Angstroms off is a very serious problem.

But a Feff model based on a crystal structure can usually do a fine 
job for amorphous systems.... 

I'd recommend looking at the simplest model (ie, 1 Feff path, no
fitting parameters at all)  in k-space.  That is, from Artemis (or
SixPack or Ifeffit), make a model with the first path and compare
that to the data chi(k) and don't vary anything in the "fit".

Doing this , the amplitudes may be off significantly -- you might
put in a fixed value for sigma2 (say, 0.005Ang^2 as a rough guess,
but you may want to play with that number) to get the amplitudes
close, but don't worry too much about it being exact. 

But now, if the oscillations are way off (off in frequency or
shifted), then the mode is not very good and you'll never get a good
fit: either the distance is way off or the species of the
backscattering atom is not right.  Also if the _shape_ of the
envelope around chi(k) is very different between data and model,
that indicates that the backscattering atom might be wrong.

Hope that helps, 

--Matt