[Ifeffit] number of independent points

[George Sterbinsky]

Bruce, Matt, and Jeremy,

Thank you for explaining this. Your responses are much appreciated.

George

On Mon, Oct 10, 2011 at 11:35 AM, Bruce Ravel <bravel at bnl.gov> wrote:

> On Monday, October 10, 2011 10:35:12 am George Sterbinsky wrote:
> > Since I haven't received a response to my previous question, I just
> wanted
> > to make sure I asked it clearly.
>
> I suspect the lack of response had more to do with it being a
> non-trivial question asked on a Friday afternoon :)
>
> > To elaborate, I am attaching a log from
> > Athena. The log file lists:
> >
> >  "Independent points          =      27.057617188"
> >
> > If I attempt to reproduce this result using the equation in the feffit
> > document, I find
> >
> > Nidp = { [2*(kmax - kmin)*(Rmax-Rmin) / pi] + 2} = { [2*(14 - 0.5)*(4.2 -
> > 1.0) / pi] + 2} = 29.50197417,
> >
> > where I have used the k-range and R-range in the log file as kmax, kmin,
> > Rmax, and Rmin.
> >
> > Can someone explain to me why may calculation does not reproduce the
> number
> > given for "independent points" in the log fie?
>
> George,
>
> A quick examination of the ifeffit source code shows that there are
> two issues:
>
>  1. Regardless of what it may say in the document, Ifeffit uses "+0"
>    rather than "+2".  See line 345 of
>    http://cars9.uchicago.edu/svn/ifeffit/trunk/src/lib/iff_feffit.f
>
>    A bit of history: Ed Stern wrote this paper back in 1993:
>       http://dx.doi.org/10.1103/PhysRevB.48.9825
>
>    Over the years, many have expressed scepticism of his conclusion.
>    The Bayesian work I mentioned earlier suggests quibbling over the
>    value of N in the "+N" term misses the bigger issue.  Ifeffit's
>    use of "+0" provides a conservative yet easily stated estimate of
>    Nidp.
>
>  2. The difference to the left of the decimal point has to do with the
>    fact that Artemis allows you to pick any value for kmin, kmax,
>    rmin, and rmax.  When Ifeffit evaluates Nidp (and anything else)
>    is actually takes the nearest grid points to those four values.
>    The grid in k-space is 0.05 inv. Angstroms.  This sets the grid in
>    R space to be about 0.03 Angstroms, but not exactly.  So the
>    values of Rmax and Rmin used by Ifeffit to compute Nidp (and other
>    things) are not quite what you specified.
>
> At some point, George, you may want to bite the bullet and look at
> source code.  Details are not unknowable, they just might not be
> written down anywhere beside the source code.
>
> Here is the top of Ifeffit:
>   http://cars9.uchicago.edu/svn/ifeffit/trunk/
>
> 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
>
>  My homepage:    http://xafs.org/BruceRavel
>  EXAFS software:  http://cars9.uchicago.edu/ifeffit/Demeter
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20111010/2888a524/attachment.html>