[Ifeffit] Fwd: Re: Use of feff8 with D-artemis?

[Bruce Ravel]

On Thursday, July 12, 2012 05:00:00 PM Matt Newville wrote:
> > I would challenge a reviewer to come up with a reliable study
> > demonstrating that Feff8 or Feff9 provides a statistically significant
> > improvement to EXAFS analysis.  Obviously, I am not saying that full
> > multiple scattering, self-consistency, and the various potential
> > improvements in Feff9 are not relevant to XANES calculations or to
> > Feff's treatment of other spectroscopies.  What I am saying is that it
> > has not been rigorously demonstrated that those things have an impact
> > on the calculation of the extended EXAFS such that the analysis is
> > improved in a statistically meaningful way.
> > 
> > In short, I find that such arguments from reviewers boil down to
> > "Feff8 is 33% better than Feff6 because 8 is 33% bigger than 6!"
> 
> I would say that there are some demonstrated cases where Feff8 is
> slightly better than Feff6 at modeling EXAFS.   The most notable cases
> are when H is in the input file -- Feff6 is terrible at this.
> 
> Feff8 has gone through many revisions, so minor version might matter
> here.  Some of the earliest versions actually gave worse fits (in
> terms of goodness-of-fit statistics) to EXAFS  than Feff6.   Versions
> around 8.2.4 (if memory serves) ended up with approximately the same
> goodness-of-fits as Feff6.  And, although few cases have been tested,
> and the process is hard to generalize,  if multi-pole loss terms are
> used in Feff8.5 or later, the improvement in goodness of fit is
> actually quite noticeable.
> 
> Whether these differences have any impact of the accuracy of fitted
> values is harder to determine, of course.


Alrighty, fair enough.  I admit to being a little vague about what the
different minor versions of Feff8 do.  And I certainly agree that the
multi-pole loss terms represent a substantial improvement for the
interpretation of EXAFS.

That said, your last sentence is the gist of my comment.

To my knowledge (and I freely admit that my knowledge may be limited
and inaccurate), no one has done a rigorous statistical assessment of
how EXAFS analysis is affected by the various bits of theory that are
in Feff8 and are not in Feff6.

It's a shame that hasn't been done.  It seems like rather low-hanging
fruit.  Regardless of the outcome, such a study would certainly have
an impact on how I develop and promote my software.




The other relevant issue here is that the specific version of Feff6
that comes with Ifeffit and Demeter is the *only* version of Feff
that can be freely redistributed.  The substantial majority of the
users of my software want a package that can be installed with one
download and one double-click.  That specific version of Feff6 is the
only one that I can guarantee will be installed correctly along with
the rest of my software.

If having routine, well-integrated access to later versions of Feff is
important to the people on this list, I am not the person you need to
talk to.  You need to bring it up with John.

B


-- 

 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

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