Re: convolution with "corrections"

["John J. Rehr" <jjr@leonardo.phys.washington.edu>]

Dear FEFF developers,

  Gilles Hug asked about possible bugs with the CORRECTIONS Vr Vi card with
FEFF, since he observes spurious behavior at the edge.

> It seems that the convolution is adding a pre-peak which is an 
> artefact. A convolution by a gaussian with another program does not 
> give such peak. (It is less handy, however, because it is necessary 
> to convert to evenly spaced data, etc..).
> Can anyone confirm if it is a bug, a incorrect use or an artefact of 
> te mathematics use by feff?

Such spurious behavior is not a "bug" per se, but rather a limitation of the
rough approximations used in the CORRECTIONS [it's not a pure convolution]
which are only meant to allow small corrections (not more than a an eV or so)
at the end of a calculation. Thus the card should only be used with caution,
as noted in the FEFF Doc.  The more precise way to add edge shifts and
broadening is with the EXCHANGE card, but this requires recalculating
the phase shifts and redoing the EXAFS/XANES calculations with a different
imaginary potential and edge shifts.

   Alex tried, but could not reproduce the specific output Gilles reported,
in particular the noticeable shift in the edge position with Vr = 0. There
shouldn't have been any such shift.  Thus we will need the feff.inp file
to probe this further. Indeed, it is important for all bug reports to include
the version number of the FEFF code used together with the feff.inp file,
so we can try to reproduce the bug.  Please see the FEFF Doc on Bug Reports.

  Cheers,
  John Rehr
  Alex Ankudinov