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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End of Ifeffit Digest, Vol 36, Issue 1 ************************************** ------- End of Original Message -------
On Wednesday 01 February 2006 13:14, Renato Canha Ambrosio wrote:
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.
That's a very interesting question. I have a few suggestions of things to think about. 1. Considering static disorder in the context of a fit with ifeffit and the einstein model is pretty straight forward. You can use a math expression like eins(temp, thetaE) + sig_static then use thetaE and sig_static as quess parameters. Those two terms might prove to be rather highly correlated, but if you do a multiple data set fit with enough temperatures, you'd have a good chance of breaking the correlations. 2. You could just float a single parameter that is used for all parts of the sigma^2, then analyze that parameter as a function of temperature once the fits at all temperatures are complete. I think that is what you mean by a "non-xafs refinement". That has the advantage of being flexible and unburdened by the requirements of doing the XAFS fit. 3. You could do the einstein model but not use Ifeffit's eins function. The formula for the eins function is given in Sevillano, Meuth, Rehr, Physical Review B 20:12 (1979) pp. 4908-4911. You could make math expressions in Ifeffit that encode that formula then use any part of it as a guess parameter. This might be a bit complicated and error-prone to implement, but is certainly possible. It has the advantage of being very general and you get to evaluate correlations between the terms you are actually interested in and the other terms in the fit. Hope that helps, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
Dear Renato, Do you mean that you'd like to refine not only heta_E, but also refine the reduced mass in the fit, with sigma2 defined as sigma2 = [ (hbar*c)^2 / 2* k_B ] * [ coth(theta_E / (2*T)) / ( theta_E * M_R ) ] + sigma2_static That seems doable (that is, you could just 'def'ine this formula for sigma2). I doubt it would be highly robust to vary all of theta_E, M_R, and sigma2_static for a simple problem, but you might be able to put other constraints on some of these parameters..... Is that what you getting at, or did I miss the question entirerly? --Matt PS: I'd double-check the formula, but I think it's right and that [ (hbar*c)^2 / 2* k_B ] ~= 97 / 4 in the units you want (that is, theta_E and T in K, sigma2 in Ang^2, and M_R in amu).
participants (3)
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Bruce Ravel
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Matt Newville
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Renato Canha Ambrosio