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
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