[Ifeffit] correlated Einstein model and Debye model

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 Open WebMail Project (http://openwebmail.org) ---------- Original Message ----------- From: ifeffit-request@millenia.cars.aps.anl.gov To: ifeffit@millenia.cars.aps.anl.gov Sent: Wed, 1 Feb 2006 12:00:12 -0600 Subject: Ifeffit Digest, Vol 36, Issue 1

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Today's Topics:

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 Subject: Re: [Ifeffit] FEFF8 vrs FEFF6 To: XAFS Analysis using Ifeffit Message-ID: Content-Type: text/plain; charset=ISO-8859-1

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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