[Ifeffit] Other programs than FEFF - GNXAS and MXAN

[John J. Rehr]

Hi Everyone,

  All the critical feedback on the various codes (and especially FEFF)
to be quite valuable, especially since the main aim of FEFF Project
is to develop improved, user-friendly theoretical tools for x-ray and
related spectroscopies.

  In my view the main difference between FEFF and codes like GNXAS and
EXCURVE and MXAN is that FEFF only does ab initio theoretical simulations, with
a minimum of adjustable parameters. The others named above also do fits,
so they are truly ab initio codes. On the other hand, FEFF requires
auxilliary codes like IFEFFIT/ARTEMIS, WINXAS, EXAFSPAK, etc.
to compare with experiment.  Fitting can, of course, even make non-sensical
theory fit experiments with enough parameters, so one cannot directly
compare fitting codes with theoretical codes.

  Although we have been interested in MEE for a long time, and know that
such excitations must exist (since S_0^2 is not unity!), we have not
introduced them into FEFF for lack of a quantitative theory. The
sudden approximation does not appear to be very accurate, for example,
due to the importance of interference terms, especially near the onset
of MEE.  However, it may be possible that the quasi-boson model
(discussed e.g. at XAFS13) can provide such a theory.

   J. Rehr