Re: FEFF question?

[Matt Newville <newville@cars.uchicago.edu>]

Hi Rich,

On Tue, 4 Sep 2001 rjreeder@notes.cc.sunysb.edu wrote:
> Hi Matt,
>
> We have a question about FEFF output files and I wasn't sure who best to
> ask.  Can I aim it at you first, and if necessary you might be able to
> redirect me to someone else.

This sounds like a perfect question for the "Feff User's Mailing List"!
I hope it's OK that I pass it along to them.

> We've been using FEFF7 but have just started to use FEFF8.1, so far just
> for basic EXAFS fitting.  We happened to notice that the phases and to a
> lessr extent the amplitudes calculated by FEFF8.1 are noticeably different
> than those calculated by FEFF7 (using the same input file and with the same
> "keywords").  In other words, when we compare the different phase and
> amplitude columns (basically cols 2, 3, and 4) in the FEFFnnnn.dat file for
> a given path, the results are different depending on whether we use FEFF8.1
> or 7.  In doing this comparison, we specifically did not select SCF or FMS,
> so the calculations should be comparable (we think).
>
> BUT, when we then use the paths to do the fitting, we find basically no
> difference in the fit results for paths from 8.1 or 7.  So, we're puzzled
> about two separate things.  First, why the difference in the phases and
> amplitudes (much smaller difference in amplitudes)?  And second, why is
> there NOT a difference in the fit results if the phases and amplitudes in
> the FEFFnnnn.dat files are different?  (Actually, there is a systematic
> difference in the fit results in E0, but nothing else).  I assume we're not
> the first to see this.  Have we overlooked something very simple?
>
> Any thoughts would be appreciated, or can you think of who I should ask
> about this?

My experience has been that the change in amplitudes was very small
(but maybe I haven't looked carefully recently), but that the phase
shifts could be fairly large -- so that changes in E0 of a few eV were
necessary.  I think that's in keeping with the changes in models for
interatomic potentials in Feff8 versus Feff7: relatively large changes
in the potential models, smaller changes in the amplitudes.

For the second question: I'm not sure why there's no difference in the
fit results.  Is it possible that some of the amplitude is just moved
from one column to another??  For feffnnnn.dat, the amplitude used is
[col 3 * col 5] (mag[feff] * red factor), and the total phase shift
used is [col 2 + col 4] (real[2*phc] + phase[feff]).

I think the general consensus has been that Feff8 doesn't improve
Extended XAFS over Feff7 (or even over Feff6 for non-polarized
calculations).  I'm still using Feff7 myself.

> Thanks and cheers,
>
> Rich
>
> __________________________
> Richard J. Reeder, Professor
> Dept. of Geosciences
> SUNY at Stony Brook
> Stony Brook NY  11794-2100
>
> Tel. (631) 632-8208
> Fax (631) 632-8240
>

--Matt

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