[Ifeffit] correlated Einstein model and Debye model

Renato Canha Ambrosio rcanha at ccet.ufrn.br
Wed Feb 1 13:14:53 CST 2006


Good afternoon everybody

I have questions about the Einstein and Debye correlated models in IFEFFIT.

As described in page 80 of the EXAFS ANALYSIS WITH FEFF AND FEFFIT - PART 2: 
COMMENTARY by Bruce Ravel the eins function is a function evaluated in the 
context of the current path. So the reduced mass is calculated with the mass 
of the atoms in the path. Its ok but I have several scans at different 
temperatures so I would like to refine not only the Theta (Einstein 
temperature) but also the reduced mass and perhaps the static disorder. Can 
I do it in the context of the path or I have to perform a non - xafs 
refinement with the file containing the temperatures and sigma^2. I make the 
same question for the Debye correlated model.
Thank you very much for your kind attention

Renato Canha
Universidade Federal do Rio Grande do Norte
Natal - RN - BRASIL

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Sent: Wed, 1 Feb 2006 12:00:12 -0600
Subject: Ifeffit Digest, Vol 36, Issue 1

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>    1. Re: FEFF8 vrs FEFF6 (Matt Newville)
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> Message: 1
> Date: Wed, 1 Feb 2006 08:54:22 -0600
> From: Matt Newville <newville at cars.uchicago.edu>
> Subject: Re: [Ifeffit] FEFF8 vrs FEFF6
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
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> 
> HI Shelly,
> 
> I wouldn't say it's wrong to use Feff8 for EXAFS analysis, just 
> rarely necessary.
> 
> Feff8 was definitely designed for XANES, and improvements for EXAFS
> were not the main goal.   But there are a few cases where Feff8 _is_
> known to be better for EXAFS than Feff6.  (I'll leave out Feff7 as it
> seems no one is using this much).  Clusters with H is one, and
> transuranic elements is another.  I've heard a few people claim that
> Feff8 gives "better E0's", which I think means that  E0 refines to a
> value closer to 0 (that is, to where the it's arbitrarily assigned in
> the data processing).  Since E0 has to be refined and has little
> physical significance, I don't find this to be a compelling advantage
> for Feff8.
> 
> Early versions of Feff8 (pre-release?  Including 8.00?? I'm not sure)
> were much worse for EXAFS.This did get fixed,  but I'm not sure when
> (I have a hard time understanding Feff8 version numbers and can never
> really tell which get released and widely used: I've given up
> complaining and live wih the fact that "Feff8" means one snapshot of 
> 8 years worth of work.).   By Feff8.1 for sure, and maybe eralier, there
> were essentially no difference between Feff6, 7, and 8 for EXAFS.  
> That was based on analysis of data on metal foils (Cu, Ag, Au), and
> could hardly be called thorough and possibly not even 'highly 
> relevent to real-world problems', as it doesn't even test changes in 
> exchange or overlap of metal-oxygen potentials.   Better testing and 
> comparisons would be very good.
> 
> Anyway, I think it's fine to use Feff6, 7, or 8 for EXAFS.
> 
> I don't think anything should change for these versions in the
> reporting of how a Feff calculation was done for EXAFS.   Most EXAFS
> work using Feff give no details about the calculation, and I think
> that's mostly OK, as the Feff/Atoms/Artemis default parameters are
> generally quite reasonable.   If the work discusses comparing
> different Feff calculations, then obviously more detail on what was
> changing would be needed.
> 
> That's my opinion, anyway.
> 
> --Matt
> 
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