Re: [Ifeffit] Breaking down correlationships between parameters

23 Mar 2015

      Hi Jatin,

The key is that S02 should be the same for all paths.

For example:

Suppose you are very confident path 1 has a coordination number of 6, because of prior knowledge you have about the system. Paths 2, 3, and 4 have unknown coordination numbers, however.

N1 (i.e. the degeneracy of path 1) you set to 6.

You guess S02, N2, N3, and N4. Of course, the way Artemis and Ifeffit implement that, you're writing N1*S02 for the S02 field of Path 1, N2*S02 for the S02 field of Path 2, etc., and then setting N1 = 6 and guessing S02, N2, N3, and N4.

There is now enough information for S02, N2, N3, and N4 to each be fitted without 100% correlation.

—Scott Calvin
Sarah Lawrence College
...
On Mar 23, 2015, at 8:54 AM, Rana, Jatinkumar Kantilal  wrote:
Hi Chris,
The term N*S02 is fitted for each path of the FEFF calculation. So my question is, even if we know N with a great certainty for some path, how can we vary both N and S02 for other paths ? or Did I understand it wrong ?
Best regards,
Jatin
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Date: Mon, 23 Mar 2015 11:00:19 +0000
From: "Rana, Jatinkumar Kantilal"

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Subject: Re: [Ifeffit] Breaking down correlationships between
       parameters
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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
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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