[Ifeffit] Possible bug in atoms files generation by Artemis

[Matt Newville]

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