Re: [Ifeffit] Breaking down correlationships between parameters
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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Today's Topics:
1. Re: Breaking down correlationships between parameters
(Rana, Jatinkumar Kantilal)
2. Re: Breaking down correlationships between parameters
(Chris Patridge)
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Message: 1
Date: Mon, 23 Mar 2015 11:00:19 +0000
From: "Rana, Jatinkumar Kantilal"
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
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request@millenia.cars.aps.anl.gov Sent: 23 March, 2015 12:16 To: ifeffit@millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 145, Issue 42
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Today's Topics:
1. Re: Breaking down correlationships between parameters (Rana, Jatinkumar Kantilal) 2. Re: Breaking down correlationships between parameters (Chris Patridge)
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Message: 1 Date: Mon, 23 Mar 2015 11:00:19 +0000 From: "Rana, Jatinkumar Kantilal"
To: "ifeffit@millenia.cars.aps.anl.gov" Subject: Re: [Ifeffit] Breaking down correlationships between parameters Message-ID: Content-Type: text/plain; charset="utf-8" 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
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of ifeffit-request@millenia.cars.aps.anl.gov Sent: 23 March, 2015 10:16 To: ifeffit@millenia.cars.aps.anl.gov Subject: Ifeffit Digest, Vol 145, Issue 41
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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
On 03/23/2015 08:54 AM, Rana, Jatinkumar Kantilal wrote:
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 ?
At no point in your emails did you say that you are using Artemis to do your fitting. The comment I am about to make may not, therefore, be relevant to you. In Ifeffit and Larch -- and therefore in Artemis -- we DO NOT fit N*SO2. Fits in Ifeffit, Larch, and Artemis use a set of user-defined parameters as the variables of the fit. As part of the fitting model, the user relates the variables of the fit to the parameters of the EXAFS equation using math expression. This benefit of the generality of the fitting model allows the user to easily encode prior knowledge into a fit. The cost is that the parameters have to be interpreted in some kind of physical context. To answer you question specifically, Artemis allows you to set or float parameters for N and S02 and to define the amplitude term of the EXAFS equation for each path in any way that you see fit. In that way, you can implement all the suggestions that Matt, Scott, and Chris have made. I discuss this in some detail starting at page 35 of this presentation. https://speakerdeck.com/bruceravel/advanced-topics-in-exafs-analysis Among the lectures at http://www.diamond.ac.uk/Beamlines/Spectroscopy/Techniques/XAS.html, this is the one called "Fit Evaluation and Fitting multiple structures". HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 535A Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/
participants (3)
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Bruce Ravel
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Rana, Jatinkumar Kantilal
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Scott Calvin