[Ifeffit] Attempted XANES calc using FEFF8.1 - what's wrong?

[Matthew Marcus]

With all the playing around one has to do to get a known structure to agree, 
how can one fit
unknown structures?  What started this whole thread was the talk I heard by 
Wu Ziyu who claimed
to fit metalloprotein structures based on subtle features of the XANES, 
while I can't even get the broad features to agree!
It makes me wonder how robust those results really are.
How, for instance, would I know that the Fermi energy needs to be shifted or 
that the exchange model might be wrong?
Presumably, other systems will require other remedies.  What would be nice 
is if some expert in XANES calculation could
write a review showing the effects of, reasons for, and indications for use 
of, these adjustments.
    mam
----- Original Message ----- 
From: "John J. Rehr" <jjr at leonardo.phys.washington.edu>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Cc: "John J. Rehr" <jjr at phys.washington.edu>
Sent: Sunday, August 13, 2006 10:25 PM
Subject: Re: [Ifeffit] Attempted XANES calc using FEFF8.1 - what's wrong?


> Hi Matthew,
>
>> I think there are more serious disagreements than just the amplitudes of 
>> two
>> peaks:
>> 1.    The pre-edge features are missing in FEFF (and FDMNES).
>> 2.    The big second feature that's split in experimental data isn't in
>> simulation.
>
>   Actually some pre-edge features and the split in the 2nd feature are
> visible in the pDOS. To see the preedge in FEFF one has to shift the
> Fermi energy by 2 eV (Vr=2) to get the Fermi energy right. Also, one has 
> to cut the loss, Vi = -.3 eV.  Matt's remarks illustrate one of
> the problems with the defaults in FEFF8, namely that the plasmon-pole
> self-energy has too much loss in the near edge. It might be worthwhile
> to compare results with another exchange model, e.g. Dirac-Hara exchange.
> or ground state.
>
>  John
>
>
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