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@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