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

[John J. Rehr]

Hi Matthew,

> With all the playing around one has to do to get a known structure to agree, 
> how can one fit
> unknown structures?

    All the structural information in XAS is in chi(k) - so it is most easily
accessible using EXAFS analysis of the fine structure. Getting structural
info from XANES is possible, but limited. For example, with MSXAN
one can get nn distance. Preedge information is more electronic than
structural.

> I can't even get the broad features to agree!
   I don't agree. I think there is actually rough agreement e.g. with
the pDOS and the XANES apart from amplitudes. The amplitudes are
wrong, but the shapes locations are not far off.

> How, for instance, would I know that the Fermi energy needs to be shifted or 
> that the exchange model might be wrong?
   You can tell by how the edge looks and by looking at the broadening.
   It's easy for anyone with a little experience to see/understand.

> 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.

   Good idea. For a little discussion see for example, ``Calculation and
interpretation of $K$-shell x-ray absorption near edge structure of
transition metal oxides," H. Modrow, S. Bucher, J. J. Rehr and
A. L. Ankudinov, Phys. Rev. B.  B {\bf 67}, 035123 (2003).

  Cheers,
  J. Rehr