November 2004 - Ifeffit - millenia.cars.aps.anl.gov

re:fits
by k-kupiecki＠northwestern.edu 15 Jan '05

15 Jan '05

Thank you Bruce and Shelley! I just wanted to thank Matt, Bruce and everyone involved with IFEFFIT/Artemis/Athena... it has been a Godsend.... I couldn't have stumbled on it at a better time (I'm frantically trying to analyze about 4 beamtimes worth of SEXAFS data for papers and my thesis and WINXAS wasn't really working out for me)!!!!! Does anyone have any suggestions for additional literature in regards to better understanding the quality of fits and the parameters involved?.... or perhaps know when we might be able to expect a updated version of the Artemis literature? (What is currently there has helped me sooo much that I've been checking for an update every couple of weeks hoping to get see sections 2.2-3.2..... ) Also.... I guess I've realized that only knowing IFEFFIT with Artemis I kinda take things for granted (like I didn't know that IFEFFIT was having problems, in certain cases since I had only been checking the "Results" and the error messages on the bottom of the window....) which meant I missed errors like this in the IFEFFIT (since I didn't notice anything unusual in the "Results"): ............ # Do the fit! ## fitting difference of Cov_0_06ML_bl1435_111703.chi and one or more paths ... ## === data set #1 of 1 feffit(1-8, group=data0_fit, chi=data0.chi, k=data0.k, kweight=2.00, rmin=1, rmax=4, kmin=2.5, kmax=15, dk=2, kwindow=kaiser-bessel, fit_space=R, do_bkg=no, data_set=1, data_total=1) feffit fitting 1 data sets fitting ... fitting ... fitting ... fitting ... fitting ... fitting ... fitting ... fitting ... no uncertainties estimated! fit gave a warning message: too many fit iterations. try again with better guesses or a simpler problem. ............................................... I guess my question is.... is there anything else I should be looking for in the IFEFFIT printout that I might not otherwise notice by just using Artemis and looking at the error messages and "Results"? Thanks for the input! Sincerely, Kristine Witkowski Northwestern University

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Athena & Artemis in Windows
by Peter Southon 22 Dec '04

22 Dec '04

Hello Windows people A few days ago Shelly suggested that Athena and Artemis don't work when installed in the directory "C:/program files/ifeffit/". This has always worked for me using XP, though I wouldn't be surprised if it caused problems in some earlier versions of Windows. Regarding Poorani's question about TRAP files, I have found it useful to put a shortcut to "C:/program files/ifeffit/horae/stash/ARTEMIS.TRAP" on my desktop or wherever for easy access. Cheers, Peter >Date: Fri, 11 Jun 2004 08:36:32 -0500 >From: "Kelly, Shelly D." <SKelly(a)anl.gov> >Subject: RE: [Ifeffit] Running FEFF >To: "XAFS Analysis using Ifeffit" <ifeffit(a)millenia.cars.aps.anl.gov> >Message-ID: <1C1577844A0BDF448E14E8CCB2F74424015A1B6D(a)ANLMAIL.anl.gov> >Content-Type: text/plain; charset="us-ascii" > >Hi Poorani, > >I believe that Athena and Artemis do not work properly when installed in >the C:/program files/ifeffit directory. The installation proceedures >say to specify C:/ifeffit instead. To fix if, but all of C:/program >files/ifeffit into the trash and then re-install. > >Shelly Peter Southon Post Doctoral Fellow School of Chemistry University of Sydney NSW 2006, Australia +61 2 9351 4425

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RE: [Ifeffit] Using more than one E0
by Kelly, Shelly D. 14 Dec '04

14 Dec '04

Hi Guys, I wanted to add another comment about using multiple E0's. One of the best formulas that I have found involves assigning different atom types different E0's. This method seems to work well for M.S. paths. Here is an example of how it goes. I think that Daniel Haskel was the first to publish this type of E0 stuff. "Single and multiple scattering XAFS in BaZrO3: a comparison between theory and experiment" D. Haskel, B. Ravel, M. Newville and E. A. Stern Physica B 208&209 (1995) 151 Lets pretend that we have a copper oxide that we want to model. First shell Oxygen called O1, Second shell Cu called Cu1, Third shell Oxygen called O2. This method would use three E0's. One for the first shell oxygen atoms, Eo1. One for all the Cu atoms Ecu. and one for all other oxygen atoms Eo2. The Cu-O1 path has E0=Eo1. The Cu-Cu1 path has E0=Ecu. The Cu-O2 path has E0=Eo2. The Cu-O1-Cu-O1 path has E0=Eo1, (only scattering from O1 atom). The Cu-O1-Cu1-Cu path has E0=0.5*Eo1+0.5*Ecu. (scattering once from O1 and once from Cu1) ... And so forth and so on. Now if you set something like this up. You need to look at the results carefully and see if it works. Generally, scattering from an atom that has lost an electron due to an ionic bond will have a positive deltaE, and scattering from an atom that has gained an electron will have a negative deltaE. Of course this negative/positive deltaE is added to the alignment deltaE. If the values for Eo2 and Ecu are the same within the uncertainties of the measurement then you need to make them the same variable, so then you will only have two energy shifts, but you can still use it. This is most often the case as Matt mentioned but I always like to test it out. In addition, one electron transfer is about a shift by 6 eV, so if these values differ by more than say 10 eV something is not right. Since the shift is generally smallish, say 2 eV, your fit has to be really accurate to "see" these shifts. In particular they seem to be most important when you have all the deltaR values constrained in some way so that the error in deltaR is really really small. For example all deltaR values are related to unit cell parameters and atomic positions or and overall expansion term. HTH Shelly -----Original Message----- From: Wojciech Gawelda [mailto:wojciech.gawelda@epfl.ch] Sent: Thursday, May 27, 2004 1:18 PM To: ifeffit(a)millenia.cars.aps.anl.gov Subject: [Ifeffit] Using more than one E0 Hi everybody, I would like to address a couple of questions which are partially related to my recent struggles in fitting some EXAFS data. I'm trying to fit my data using several shells of different neighbors including a few single scattering paths and also so multiple scattering contributions (mainly collinear multiple scattering paths) all calculated with the help of FEFF 8.20. Now, I found once in the FEFFIT manual the following suggestion: one might consider using several different E0's for different paths in order to improve the fit. Ok, the explanation was based on some approximations coming from FEFF code which include incomplete core-hole shielding, lack of angular variations of the valence charge distribution and charge transfer between atoms in polar materials. My question is the following: does anyone of you have some experience with such procedure? And if yes, shall than distinguish between the first shell of nearest neighbors and the rest of the atoms in terms of their E0 corrections (using 2 parameters)? Or perhaps one can use separate E0's for each path? That's one thing... The second question concerns the background correction in Artemis: I was trying to find an easy explanation of what this procedure does during the fit but I guess I'm still somewhat confused...how one can judge whether the spline co-refinement does the right job? Some people on this mailing list underline that one should avoid high correlations between fitting parameters and background parameters in order to make sure that the fit is correct. But is it the only criterion? In some of my fits background line looks kind of constant over the entire R-range which was taken into the fit. Sometimes it becomes like a modulation peaking underneath some scattering paths...even if the background parameters don't correlate significantly with fitting parameters I have my doubts whether the result is trustful. Any hints about this procedure? Well, I'll greatly appreciate any comments and feedback from all you guys. I hope I'll be able to share some of my own soon :-) Cheers, Wojciech ********************************************************************* Wojciech Gawelda Laboratoire de Spectroscopie Ultrarapide (LSU) Institut des Sciences et Ingénierie Chimiques (ISIC), Faculté des Sciences de Base (SB-BSP) Ecole Polytechnique Fédérale de Lausanne (EPFL) CH-1015 Lausanne-Dorigny, Switzerland Tel.: +41 (21) 693 0452 Fax.: +41 (21) 693 0422 E-mail: <mailto:wojciech.gawelda@epfl.ch> wojciech.gawelda(a)epfl.ch WWW: <http://lsu.epfl.ch> http://lsu.epfl.ch *********************************************************************

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Importing Athena data into Excel
by sej9＠cornell.edu 29 Nov '04

29 Nov '04

I can't remember how to import chi(r) data from a merged file in Athena into Excel. Sarah

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formula checking
by Peter Southon 22 Nov '04

22 Nov '04

Hello all, I've got a quick question about the checking of formulas. I mistakenly typed "//" instead of "/" into several 'def' commands in my model. Unfortunately it took me a long time to find the mistake, since it was a complex model and by some fluke the equation gave almost (but not quite) the expected value. Since "//" doesn't seem to be a valid Ifeffit operator, why didn't either Ifeffit or Artemis pick it up? Regards, Peter Peter Southon Postdoctoral Fellow School of Chemistry University of Sydney NSW 2006, Australia +61 2 9351 7482

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Rbkg value in Athena
by Ann Liang 18 Nov '04

18 Nov '04

Hi Bruce, I am a little bit confused on what the Rbkg value should be when I do the background subtraction. I have read some previous posts which stated that Rbkg can be determined after the fit. However, I always think that the fit depends strongly on how the background being subtracted. How does the initial spline value affect the background subtraction? Should it be 1eV or 25 eV above the edge? I appreciate your help. Thanks, Ann

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Re: [Ifeffit] formula checking
by Scott Calvin 17 Nov '04

17 Nov '04

Hi Matt, Bruce, etc., I think Matt's suggestion is a very good one. In the past, I have occasionally run into problems where Ifeffit flags a problem but Artemis doesn't check for the particular flag. Since when I'm rolling along using Artemis I don't alsays inspect the Iffefit messages, I didn't realize anything funny has happened immediately...it wasn't until I realized the fit didn't make sense that I checked if Ifeffit had correctly identified a typo or whatever. I've always alerted Bruce when these things have come up, and he's incorporated them into new releases, but it would be very good if Ifeffit had a couple of generic warning flags that Artemis could grab on to and thus prominently alert the user that something went wrong... --Scott Calvin Sarah Lawrence College At 11:08 AM 11/17/2004 -0600, you wrote: >Hi Bruce, Peter, > >Ifeffit does print a warning message about a syntax error on using >'//'. But it's completely believable and forgivable that Artemis >doesn't catch this. > >Bruce, would it help if a flag was set indicating 'syntax error' >in addition to printing the warning message? That way you could >wrap each of the definitions with a perl function that did the >ifeffit command and check for syntax errors, doing something >sensible (popping up a warning dialog box, for example) on a >syntax error. That might be better than checking for '//' itself, >because it would detect other syntax errors as well. > >A flag meaning 'math error' (as on divide-by-zero or sqrt(-1)) >might be useful too, I suppose.... >

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Re: strange distance obtained using atoms
by Bruce Ravel 15 Nov '04

15 Nov '04

On Thursday 11 November 2004 9:11 am, Valmor Mastelaro wrote: > Bruce, I´m using the atoms program to obtain the feff.inp file for a PZT > compound. when I introduced the crystallographic informations on the atoms, > I obtained only one distance that is too short to be true, Ti-Zr = 0.483 > > I verify all the crystallographic parameters in the literature and they > seens to be correct. (Journal of Physics: condensed matter 10, 6251-6269 > (1998). Hi Valmor. I am taking the liberty of forwarding my answer to the Ifeffit mailing list as well as the feffusers list. Here is the Atoms input file that Valmor sent: space = R 3 c a = 5.78510 c = 14.2880 core = Ti edge = K rmax = 9.0 atoms ! elem x y z tag occ. Pb 0.00000 0.00000 0.27170 1.00000 Ti 0.00000 0.00000 0.03590 1.00000 Zr 0.00000 0.00000 0.00210 1.00000 O 0.16321 0.34281 0.08333 1.00000 Actually, the one very short atom was only the most obvious of a host of problems with this input data. Look also at the Zr/Ti coordination shell. There are far too many atoms and they are much too close (about 0.483 AA apart -- imagine that!) to one another. Reading the paper that he cited, I see that coordinates are given for the anion and cation positions by parameterized deviations from the symmetry positions of the space group. It's really a very clever paper -- the parameters of the structure are things like cation displacement and octahedral tilt. Very cool stuff, all of which is explained abstractly in Table 1. Then in Table 3, the authors give the actual values of the parameters for each of the anions and cations. Among the data in this table are the fractional occupancies of Zr and Ti along with separate displacement parameters for those two cations. Before I go on, you need to read this old entry to the Ifeffit mailing list: http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2002-July/000092.html That entry actually answers your question about why Atoms isn't working as you anticipated, but I'll expand on a few points here. Feff (and therefore Atoms) presumes that you have one entire atoms at any given site. Feff does not allow you to fractionally occupy a location in space. This may be a shortcoming of Feff, but there are good reasons why Feff behaves that way. Since Feff does not know how to compute a cluster with fractional atoms, Atoms does attempt to write fractional atoms to the cluster. Thus, when you put *both* Ti and Zr in the atoms.inp file, Atoms put six of each in the unit cell. (This crystal has six formula units in each unit cell.) That's obviously incorrect. The paper you cite clearly states that the samples have between 13 and 40% Ti according to the stoichiometry Pb Zr_{1-x} Ti_x O_3. So it is simply incorrect to have both the Ti and Zr entries in your atoms input file. As I explain in the link above, you generally cannot solve a problem like this with a single Feff calculation. You simply must consider an analysis strategy which involves at least 2 and possibly many more Feff calculation. Are you doing EXAFS analysis? If so, then you should consider running Feff once without the Ti site and using the parameter given in the cited paper for the Zr site. Then run Feff again with the Zr site and using the parameters given for the Ti site. Then, when you do your analysis, you will need to multiply the amplitude terms for the paths from the Zr calculation by (to use the example of one end member) 0.87 and multiply the amplitudes of the paths from the Ti calculation by 0.13. Or are you doing XANES calculations? In that case, you will need to run many Feff calculations with the Ti and Zr sites randomly populated following the modle of the Au2O3 that I discuss in the link above. In any case, the bottom line that you have to remember when using Atoms and Feff is that these programs do not and cannot treat occupancy in the same manner as is discussed in a paper on Reitveld refinement. (That is true at least in the context of running a *single* Feff calculation.) You have to understand and think hard about how Atoms works and how Feff works and how they should be used for your specific analysis problem -- this is particularly true in a problem like this, which involves a rather complicated scheme for treating the relationship between stoichiometry and structure. Hope that helps, B -- Bruce Ravel ----------------------------------- ravel(a)phys.washington.edu Code 6134, Building 3, Room 405 Naval Research Laboratory phone: (1) 202 767 2268 Washington DC 20375, USA fax: (1) 202 767 4642 NRL Synchrotron Radiation Consortium (NRL-SRC) Beamlines X11a, X11b, X23b National Synchrotron Light Source Brookhaven National Laboratory, Upton, NY 11973 My homepage: http://feff.phys.washington.edu/~ravel EXAFS software: http://feff.phys.washington.edu/~ravel/software/exafs/

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Please accept my apology
by Firouzeh Tannazi 11 Nov '04

11 Nov '04

Dear Ifeffit Mailing-List Members; Please accept my apology for accidentally sending a personal message to the ifeffit mailing list. Best Regards, Firouzeh Tannazi.

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Thesis Correction
by Firouzeh Tannazi 11 Nov '04

11 Nov '04

Hi Dr. Bunker; Let me at first thank you very much for the corrections and edition that you have made on the dissertation. I just want to remind you those sections that need more corrections. First the Introduction section which is very disperse! As we have "Abstract" part and any chapter has introduction we may eliminate the introduction chapter and add its stuff to other introduction parts or if you think it is better to keep it, I really appreciate if you write it in the way that more emphasize will be on last part . The next part is section 2.6, "X-ray Fluorescence Spectroscopy" which I want to talk about the differences and similarities between that method and XAFS. Regards, Firouzeh.

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