Re: [Ifeffit] Breaking down correlationships between parameters

On Sun, Mar 22, 2015 at 12:44 PM, Scott Calvin wrote:

One side-comment from me:

On Mar 22, 2015, at 12:52 PM, Matt Newville wrote:

N and S02 are always 100% correlated (mathematically, not merely by the finite k range).

Matt is saying that N and S02 are always 100% correlated *for a single path*. But in some situations you might know N for one path but not others. For example, you might know that the absorbing atom is octahedrally coordinated to oxygen but not be as certain as to next-nearest neighbors, or that there are copper atoms on the corners of a simple cubic lattice with a mixture of atoms at other positions. In cases like that, both N for all paths but one and S02 can be fit without 100% correlation.

Yes, I completely agree with Scott -- this is a good point that I neglected. In addition to looking at multiple shells, one might also consider using temperature or pressure dependence to separate N*S02 and sigma2. Those aren't without assumptions, and still don't remove the inherent correlation, but are useful approaches. The degeneracy of multiple-scattering paths can often be constrained in

terms of the coordination numbers for direct-scattering paths, which can further reduce (not “break”) the correlation.

In terms of the main question, I agree with Matt: I don’t think there’s much point in using the line-crossing technique nowadays; fitting using multiple k-weights simultaneously accomplishes the same thing but is a bit easier to interpret statistically.

—Scott Calvin Sarah Lawrence College

_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

--Matt