[Ifeffit] Possible bug in atoms files generation by Artemis
newville at cars.uchicago.edu
Tue Mar 26 23:24:28 CDT 2013
On Tue, 26 Mar 2013, Matthew Marcus wrote:
> Just to put my bit in, I believe that the most significant advantage of
> higher FEFF versions for EXAFS analysis is that it results in
> more reasonable values for E0 for high-Z elements. I forget whether the
> issue is high-Z scatterer or absorber. If you use any of the
> common prescriptions for defining E0 with, say, Pt metal in FEFF6l, your fit
> will want large values of enot. That said, I have not done
> a real test by comparing FEFF8 and FEFF6 paths. Has anyone done that? It
> would be interesting to know what happens if you simulate a
> k^n*chi(k) with one program and fit it with the other.
It's been a very long time since I've tried, but, yes I've made
such comparisons in the past, as well as comparing Feff6 and
Feff8 to the same "very good data".
Feff 8 actually has a long history. Initially, EXAFS was
noticeably worse with Feff8, but it got better over the years to
the point where I think it's hard to say that Feff8 is worse
than Feff6 for EXAFS. As you say, E0 is better (though still
needs refinement), as is S02. Feff8 also appears that it is
better for heavy elements (perhaps Z > 50, but I'm not sure
anyone has looked at that carefully). But: the multi-pole
self-energy introduced around Feff8.5 or so can make a very
large improvement for the EXAFS. Whether this can be made
generally applicable is a separate question.
Just to echo some of Bruce's frustration and build on that (and,
speaking only for myself): Basically, we're stuck with Feff6
because we do not have access to Feff8. Last I heard, John and
Josh were working on this, so that Feff8-for-EXAFS would be made
freely available. I haven't seen the code yet, but I'm
optimistic that it will be released someday.
Once this happens, I'll happily start incorporating this into
Larch. I'd very much like to replace the pathfinder (as Bruce
has done in Perl for Demeter) so that distortions are easier to
track, and allow the EXAFS calculation for a Path to be done
automatically inside the fitting loop. That will be some real
work (anyone out there interested in helping?), and could take
awhile, but could actually make a difference for modeling.
I'm pretty sure that getting the multi-pole self-energies more
universally useful would be a big help, but I think there still
some unknowns there (basically -- how well do you have to know
the dielectric response for a general system?) that have to be
worked out. Getting 1/epsilon for Cu metal is one thing (a
first step!), but getting it for As sorbed onto ferrihydrite or
would be more challenging....
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