This all suggests that Ziyi Wu's claims to be able to tell the difference between 4 and 5 waters of hydration in the metal center of a protein just from fitting XANES features may be a bit overblown. I tried again with FEFF8 and still don't see the pre-edge features, nor get the ratios right. Maybe they're visible in the pDOS, but that's not what you measure. FDMNES seems to come closer to getting the ratios right, though this may be a fluke. I'll read the Modrow paper and see how they do it. mam John J. Rehr wrote:
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 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit