[Ifeffit] number of independent points
George Sterbinsky
GeorgeSterbinsky at u.northwestern.edu
Mon Oct 10 13:04:07 CDT 2011
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
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