[Ifeffit] feff6L and path finder limit

Bruce Ravel bravel at anl.gov
Thu Aug 4 21:15:55 CDT 2005

> First of all, kudos to Bruce and Matt on the new versions. One of my
> summer students more than once uttered the phrase "This is
> <italic>awesome</italic>" as she tried out yet another new feature. I
> concur. :)

Awwwww..... now I'm blushing.....

> In my opinion, the path finder limit in feff is high enough for most
> applications. I frequently fit up to 5 angstroms, which, considering the
> difference between reff and r and the width of EXAFS peaks, means I feel
> I have to include paths up to about 6 angstroms. But that means I need to
> have feff compute a cluster bigger than that (as has been discussed on
> this list a few times)...which makes a 7 angstrom cluster seem
> reasonable; maybe 8 for safety. I have never encountered a path with more
> than 4 legs that I found to be significant.
> 2) Does anyone out there run into cases where paths with more than 4 legs
> are significant? If not, Artemis should just stick the card NLEG 4 into
> the feff.inp file automatically...that's probably a better default value.
> Of course, a knowledgeable user could still override that to include more
> legs if they wished.

The NLEGS suggestion is a good one.  I'll probably take you up on it.

The fix is very simple and is one that anyone can implement immediately
without modification to the artemis source code.  Search among the files
installed as part of my software for a file called "feff.atp".  This is
template that Artemis uses to write input data for feff.  In short, it
provides a sort of outline for what the feff.inp file should look like.
Simply add a line that says

  NLEG  4

somewhere in that file.  On a unix machine, you will probably need to copy the
feff.atp file to ~/.horae/atp/ before editing it.

> 1) Artemis now does a decent job of flagging users when iffeffit
> encounters a problem. Perhaps it should do the same for feff. (I know the
> feff error messages are often somewhat obscure. But if Artemis simply
> stated that "FEFF has encountered a possible problem" and advised users
> to check the messages screen, the behavior might seem a tad less
> baffling.)

Actually, I have been building a database of feff6 error messages.  When Feff
is finished, artemis parses the text in the window where the feff output is
displayed.  If it recognizes the error message, it displays an explanation in
gaudy red-on-white text at the end of feff's run time messages.

If you find a feff error for which Artemis does not provide an explanation,
send me a project file that triggers the error so I can add it to the database
for the next release.  I am only making responses to the error messages from
feff6L, the version that comes with Ifeffit, at this time.  If you use another
version of feff, you are on your own for now as far as recognizing what feff's
error means.

I have also been auditing the code this week to make sure that the messages
window rises to the top of the screen when an informational message of this
sort gets displayed.


 Bruce Ravel  ----------------------------------- bravel at anl.gov  -or-
                                                  ravel at phys.washington.edu

 *** My cell phone number has changed.  Please ask if you need the new number

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 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
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 My homepage:    http://feff.phys.washington.edu/~ravel 
 EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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