[Ifeffit] number of independent points

[Bruce Ravel]

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