Some users do have FEFFx (x>6l) on their own, so it would be useful to prepare
Artemis/Demeter/Larch... for them and provide
methods for using higher versions if an executable is present.
What does the multipole self-energy do? Is that the thing that requires the
dielectric response? As you point out, the purpose of
the exercise is to analyze unknowns, so by definition one doesn't have the
dielectric response. We can't expect a program that runs on
a PC to do a proper, all-electrons, excited-state calculation.
One thing I do is to use experimental data from models as sources of amplitudes
and phases. At present, I do this using my own
multishell fit program. Is there an easy way to do this in your programs?
What I think would be nice is a subsystem which allows one
to do the filtering and extraction of amp and phase from a model .chi file from
within the program, rather than having to create
FEFF-path-like files.
As long as we're talking wish-list, I'd love to have some way of defining atom
positions using symbolic variables and have the system
compute the path distances automatically as functions of those variables. That
way, I could, for instance, define a dopant system in terms
of the displacements of the near-neighbors without having to do the geometry by
hand, which is difficult, tedious and error-prone.
mam
On 3/26/2013 9:24 PM, Matt Newville wrote:
Hi Matthew,
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.
mam
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....
--Matt
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