[Ifeffit] XANES

[John J. Rehr]

  Regarding XANES fits, I'm glad to hear Bruce is working on 
adding a principal component analysis  to IFEFFIT. 

  Doing XANES fits ala Benfatto and Della Longa would also
be desirable in the long run, i.e., fitting the XAS(x) with x
standing for various parameters like interatomic distances.
To do that would generally require looping over the FMS module in FEFF
and could get time consuming. To avoid this, one needs something like
a tensor XANES calculation, which computes d mu/ dx via multiple  XAS
calculations and hence permits fast linear interpolations. This is not
implementable with IFEFFIT/ATHENA at present as far as I know. However,
I don't see major difficulties in implementation (?).

  There are some other ways to go, including using the parallel version of
FEFF8 (Bouldin for example has suggested using this approach for XANES fits).
Also, one might even consider adding constraints ala Krappe and Rossner's
Bayesean-Turchin approach for EXAFS analysis.


  John Rehr

> From B. Ravel ...

>My program, Athena, currently has the capability of fitting lineshapes
>(i.e. an arctangent and a number of Gaussians or Lorentzians) to
>normalized XANES data.  In the not-too-distant future, Athena will be
>able to fit a linear combination of standards to a XANES spectrum and
>be able to do principle component analysis (PCA).
>