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

[Matt Newville]

Hi Bruce,

On Thu, Jul 12, 2012 at 4:04 PM, Bruce Ravel <bravel at bnl.gov> wrote:
>
> On Thursday, July 12, 2012 05:31:19 PM you wrote:
>>
>> I wonder why the new Artemis (from Demeter) is limited to feff6 or feff7 as
>> the old ifeffit can deals with feff8 and feff9.
>>
>> Is it a wish? When you published data using feff7, the reviewers first note
>> that you should have used more actual version.
>
> 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.

> Is it planned that D-Artemis will work with feff8 too?
>>
>> I am using the other programs of Demeter but still have to stick to the old
>> Artemis as none of my files can be read by D-Artemis (always error due to
>> feff8 or feff9 files by importing old project)

This is unfortunate, but I'm willing to stick up for Bruce on this.
Feff8 (I have to admit I do not have a copy of Feff9) is really a very
different program from Feff6.     Although the Feff team does great
physics, the several different calculations that Feff8 can do are
quite difficult to work with in any automated way.   Artemis goes
through great effort to hide just how awful Feff6 and Feff8 are in
this respect.   Inevitably, there are subtle differences between the
inputs and outputs of Feff6 and Feff8, and being able to support all
variations in what Feff8 might be able to do is a tall order.

Asking why Feff from around 2012 cannot write EXAFS outputs that are
compatible with Feff code from around 2000 or 1996 is a reasonable
question.  Sadly, Bruce (nor I) have any influence on this.

All said, Bruce's approach of Feff8 not bothering to support Feff8 for
EXAFS seems completely reasonable to me.  If the Feff team were
interested in seeing Feff8 being used in DArtemis, they would have to
be the ones to make that happen, as they have refused to let Bruce or
I work on this.

--Matt