[Ifeffit] FEFF report: Hard tests failed in fovrg.

[Bruce Ravel]

On 08/06/2013 11:55 AM, Naumova, Maria wrote:
> May I use this FEFF calculation as a valid base for future fit? Or
> this error means that FEFF doesn't work correct and I can't rely on
> its output? If the phase-shift program failed to converge does it
> mean that it could stop in some completely unrealistic result?


Maria,

You are correct that the version of Feff6 that we are allowed to give
away for free reliably complains about failing something called a
"hard test".  This is some kind of convergence test on the computation
of the muffin tin potential.

The test is made in the lines just prior to this:

https://github.com/newville/ifeffit/blob/master/src/feff6/fovrg.f#L158


The error is reported here:

https://github.com/newville/ifeffit/blob/master/src/feff6/phase.f#L127


If you can make heads to tails out of the calculation in fovrg.f, you
are vastly smarter than me, vastly more patient than me, or both!

I have 2 comments on the main point of your post:

  1. As you noted, this question has been asked many times before.  Not
     once has anyone from the Feff project (i.e. anyone who might
     actually have a working knowledge of that bit of code) bothered to
     comment.  It would be lovely to hear from one of them.

  2. This very version of Feff has been included with Ifeffit and with
     the packages I build for my software for years.  Over a decade, in
     fact.  In that time, Feff, Ifeffit, and my software have been used
     for defensible data analysis thousands of times and by hundreds of
     practitioners.

     That does not mean that any part of the software stack is actually
     correct.  But it does mean that lots of article writers and lots
     of article reviewers have accepted the results coming from this
     stack of software.

     That does not mean that you should accept it.  Quite the contrary
     -- you would be wise to question every part of the software stack.
     You may even find that you will need to discard any or all parts
     of that software stack and replace them with something you trust
     more -- perhaps even with something that you, yourself have
     written.

To summarize, I am saying the same thing I have said in the past.  I
don't understand the code that generates that message.  No one from
the Feff project has ever bothered weighing in on what it means.  No
one has demonstrated that it represents an actionable problem.  The
codes which use Feff have been in use for years to produce defensible
science.

So, in conclusion, what should you do?  I have no idea.  My advice is
to continue using the software, but my advice may not be any more
reliable than the software itself.

I hope that helps.  Probably doesn't, but it's the best I can do.
B


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

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
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  Upton NY, 11973

  Homepage:    http://xafs.org/BruceRavel
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