[Ifeffit] Breaking down correlationships between parameters

[Matt Newville]

On Sun, Mar 22, 2015 at 12:44 PM, Scott Calvin <scalvin at sarahlawrence.edu>
wrote:

> One side-comment from me:
>
> On Mar 22, 2015, at 12:52 PM, Matt Newville <newville at cars.uchicago.edu>
> 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
>
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>
--Matt
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