Re: [Ifeffit] Breaking down correlationships between parameters

23 Mar 2015

      Hi Scott,

Thank you for your comments. Can you please elaborate a little bit more on this "In cases like that, both N for all paths but one and S02 can be fit without 100% correlation."

Best regards,
Jatin

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Today's Topics:

   1. Re: Breaking down correlationships between parameters
      (Scott Calvin)
   2. Re: Breaking down correlationships between parameters
      (Matt Newville)
   3. Re: Breaking down correlationships between parameters
      (Rana, Jatinkumar Kantilal)

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Message: 1
Date: Sun, 22 Mar 2015 13:44:28 -0400
From: Scott Calvin 
To: XAFS Analysis using Ifeffit 
Subject: Re: [Ifeffit] Breaking down correlationships between
	parameters
Message-ID: 
Content-Type: text/plain; charset="utf-8"

One side-comment from me:

On Mar 22, 2015, at 12:52 PM, Matt Newville mailto:newville@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.

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