[Ifeffit] Generating single scattering paths in Artemis

[scalvin at slc.edu]

Hi Carlo,

Hmmm...my initial reaction was that, rather than loading Artemis with a
new option, it's better just to "fake it" by using a low-symmetry space
group with a large lattice parameter and then truncating the cluster size
at smaller than the unit cell. But I just tried this, and discovered an
interesting problem, which you may have come across as well.

What I did was attempted to create an octahedral cluster by using a space
group of P m 3 m (simple cubic) with a=10, an Fe at 0 0 0 absorbing, and
an oxygen at 0.2 0 0 (thus an Fe-O distance of 2 angstroms). The cluster
size is set at 5. As expected, that generates an octahedral cluster of
oxygens around the Fe as a FEFF.INP file. But here's the problem: Artemis
includes a potential for scattering atoms of Fe, and there aren't any in
the list. FEFF, in turn, doesn't like that and won't run, causing Artemis
to give the error message:

" Feff 6L.02
   No atoms or overlap cards for unique pot     1
  Cannot calculate potentials, etc.
 Fatal Error: at RDINP


      This error is probably due the Atoms list in the
      the Feff input file being too short to contain an
      example of each unique potential.

      Try increasing the Rmax on the Atoms page and
      re-running both Atoms and Feff."

This can be fixed by editing the feff.inp file to remove the Fe potential
1, but that's not enough--the oxygen scatterers then have to have their
potential renumbered to 1 to make things work. That's a lot of work to do
what should be simple.

So I vote for changing the way Artemis works in such circumstances so that
less editing is needed: if the absorbing atom is not present in the
scattering list, then it shouldn't have a second entry in the potential
list.

As far as the other special arrangements (tetrahedral etc.), if you're not
interested in going beyond nearest neighbor direct scattering EXAFS
analysis, then there is no difference between them except for a degeneracy
factor which can easily be inserted later in the process (i.e. only one
FEFF path is needed).

--Scott Calvin
Sarah Lawrence College
>
> Bruce:
>
> Occasionally, I don't know or don't care what the extended
> crystallographic environment of the absorber.  All I want to do is