[Ifeffit] errors in bond lengths

[Anatoly Frenkel]

Matthew,

In the paper by Purans et al that George references, they showed that the
FEFF fitting method and ratio method agreed, and in the ratio method the
amplitudes and phases are extracted from experimental standards, just as in
your paper.

Anatoly




On Sat, Apr 11, 2020 at 2:21 PM Matthew Marcus <mamarcus at lbl.gov> wrote:

> The uncertainties reported by Artemis include as 'noise' the systematic
> deviation of FEFF calculation from the real thing.  Even if FEFF were
> perfect, those FEFF calcs haven't been through the mutilations inflicted
> by data reduction, such as spline fitting.  What I've done when looking
> at closely-related systems such as the same thing at differing
> temperatures is shell-by-shell fitting with one of them as reference.
> For an example of this kind of thing, see:
>
> Aruguete, D. M., Marcus, M. A., Li, L.S., Williamson, A., Fakra, S.,
> Gygy, F., Galli, G, A, Alivisatos, A. P. (2007)  "Surface structure of
> CdSe nanorods revealed by combined X-ray absorption fine structure
> measurements and ab Initio calculations", J. Phys. Chem. C 111(1),75-79
> and supporting information
>
> The SI has details about how the fits were done.  In this case, the
> different spectra didn't refer to differing temperatures but rather
> different polarization directions.
>         mam
>
> On 4/11/2020 10:40 AM, George Sterbinsky wrote:
> > Hello,
> >
> > As is well known, EXAFS is more accurate at determining relative changes
> > in bond lengths than absolute changes in bond lengths due to cancelation
> > of systematic errors in relative comparisons. When comparing the
> > relative changes in bond lengths determined from EXAFS fits, as one
> > might for a temperature series for example, is it appropriate to use the
> > uncertainties returned by Artemis?
> >
> > My question arises in part from Phys Rev Lett vol. 100 pg. 055901
> > (2008), where the authors state that errors for changes in bond length
> > in a temperature series were determined by empirical means rather than
> > statistical means. This then raises the question as to if the authors
> > believe that statistical means would overestimate the error. My
> > inclination is to think that the uncertainties reported by Artemis would
> > be appropriate because of the scaling by the square root of reduced chi
> > squared. However, I want to see what others think about this before
> > committing to it.
> >
> > Thank you,
> > George
> >
> >
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