# [Ifeffit] number of independent points

Bruce Ravel bravel at bnl.gov
Mon Oct 10 10:35:59 CDT 2011

```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

```