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

mailing list downtime
by Matt Newville 20 Apr '07

20 Apr '07

Hi Folks, Sorry to bother you with non-XAFS related mail, but I wanted to let you know that the Ifeffit mailing list, web pages (everything on http://cars9.uchicgo.edu, http://millenia.cars.aps.anl.gov, and http://xafs.org) will be unavailable tomorrow (Apr 21), starting around 6am Central Daylight Time (11am UTC) and lasting for around 12 hours. This is due to a scheduled complete power outage to the building housing the server and network equipment. I assume the network equipment and server will come back from the power outage just fine, but will be watching for signs of life. Mail sent to the mailing list during the down time should queue up until the server is back up, but I wouldn't want to guarantee that mail won't get bounced over the weekend. Sorry for the inconvenience, --Matt

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Several Questions
by Edward L. Kunkes 20 Apr '07

20 Apr '07

Hello everyone, I am currently using IFEFFIT to fit LIII edge data collected for Pt, Re and PtRe bi-metallic Nan particles supported on carbon, and I have several questions: 1. I get an So2 value of 0.54 from a first coordination shell fit on Re-foil (this So2 value also seems to work when fitting NH4ReO4-) Has anyone ever worked with Re before and gotten a similar value for So2 ? 2. I model the path for the a single coordination shell as 6 octahedrally arranged nearest neighbor atoms surrounding the absorber atom at a distance equal to that of inter -atomic spacing in the metal. When comparing the IFEFFIT predicted EXAFS for this model to one that is generated by using the first path from a crystallographic model, the two seem slightly different. The nanoparticles are very small (amorphous to XRD), and probably consist of a disordered phase, so I cannot assign a crystal structure. What should I do in this case? 3. I often get high correlations the coordination number and ss, or delEo and dR (the correlations are ~0.8). Using multiple k-weights doesn’t decrease the correlations. What is an acceptable value for these correlations? Thank you very much for taking the time to answer my questions -Ed Kunkes

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Re: [Ifeffit] Structures with multiple crystallographic sites
by Vadim G Palshin 20 Apr '07

20 Apr '07

Matt, Bruce - thank you! I believe I found the problem - Artemis does seem to choke even on 200 paths, especially with the "fit background" option on. At least it does on my office PC - relatively old and weak machine, will start begging for a new one shortly. Sometimes it would generate a long error message starting with "use of uninitialized value in abs at artemis line 12984."; sometimes there is no error message but results would look really strange. Had me so confused that I started to doubt my fitting method. I have since trimmed down the number of paths and variables to a minimum (didn't need that many anyway), and the problem seems to be solved. Thanks again for your responses, gentlemen! Vadim.

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Structures with multiple crystallographic sites
by Vadim G Palshin 20 Apr '07

20 Apr '07

>Do a Feff calculation with each unique Co site as the absorbing atom, >then use paths from each calculation, weighted by the fractional site >occupancy. >Cheers, >--Matt Thanks, Matt! Yes, that's exactly what I tried doing. It's a simple cubic lattice with 20 atoms per unit cell, 8 of one type (Co1) and 12 of the other (Co2). So I did two FEFF runs in the same Artemis project, with Co1 and Co2 as the core atom, and multiplied their respective amplitudes by 0.4 and 0.6. Then I noticed that if I calculate a sum of all included paths I get a different FT spectrum than if the sums are calculated separately for the two FEFF runs and averaged (with correct weighting) outside of Artemis. Of course, it is a bit late here and I could be missing some obvious mistake. On the other hand, I am trying for 1.6-5.5 angstrom fitting range, which involves 70+ paths for Co1 core and ~120 for Co2. Is it possible that Artemis does not like to deal with too many paths and/or variables? I better go check the manual... :o) Regards. Vadim.

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Structures with multiple crystallographic sites
by Vadim G Palshin 19 Apr '07

19 Apr '07

Hello, everyone! I am trying to fit various metallic Co spectra (nanoparticles) and one of the structural models, the epsilon-Co phase, has two unique crystallographic sites for Co. How does one tackle fitting a dataset with multiple crystallographic sites? Thanks! Vadim.

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question concerning Hephaestus
by I.Reitz 17 Apr '07

17 Apr '07

Dear list members, can you please tell me, what the ion chamber utility calculates. I did not understand the perl code. Does it give me first the percentage absorbed via Beers law und then give me the Nbofphotonsabsorbed, that means the flux absorbed in the chamber or is the result that gives the program out the "number of transmitted photons" after the chamber? Thank you Irmi -- ********************************************* I. Reitz German Cancer Research Center DKFZ, E0401 Dept. of Medical Physics in Radiation Therapy Im Neuenheimer Feld 280 D – 69120 Heidelberg Phone: +49 - 6221 42-2455 Fax: +49 - 6221 42-2572 Email: I.Reitz(a)dkfz.de *********************************************

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Doubts
by Shiju N. R. 16 Apr '07

16 Apr '07

Hi, I am Shiju and I work as a research associate at the Chemical Engineering department of University of Cincinnati. We are working on the conversion of light alkanes using mixed oxide catalysts. We are trying to understand the structure of a Mo based mixed oxide catalyst that is highly active for this reaction using XAFS and have some data of this catalyst, collected at NSLS. Since I am new to XAFS, I am trying to learn it myself. I am using Athena and Artemis. I am trying to use MoO3 as a model compound for understanding the Mo local structure in my catalyst. By following Scott Calvin's ZnO example (which is highly helpful for beginners like me), I was able to get a reasonable fit for MoO3 structure. For the next step (fitting my Mo catalyst), I would like to seek help regarding the procedure I should follow. I would like to describe my understanding to approach the fitting of my Mo catalyst and please let me know if I am right or wrong. I am going to take same MoO3 structure to form the theory and import the same paths which were used to fit model MoO3 structure. Then I will set the values of S02 and sigma2 to that obtained for MoO3 and allow E0 and delr to vary during the fitting. Please let me know if this is right approach. I tried once this way, also introducing the concept suggested by Bruce Ravel to find the coordination in one of the earlier posts. I used the expression S02 = ni*amp, where i represents each path, keeping the degeneracy of each path as 1. I was able to get a fit, however, ni are fractions and their sum is much less than 6, while I am expecting an octahedral coordination of Mo in my catalyst. Is it due to any mistake in my approach? what should I do to correct it? Please excuse if these questions seem silly... I am just a beginner in XAFS! Shiju Dr. Shiju. N. R. Research associate Chemical and Material Engineering University of Cincinnati Ohio USA --------------------------------- Yahoo! Mail is the world's favourite email. Don't settle for less, sign up for your freeaccount today.

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Platinum Rhenium EXAFS in Artemis
by Edward L. Kunkes 16 Apr '07

16 Apr '07

Hi everyone, I am new to EXAFS and have been working with Athena to process my data and trying to fit it with Artemis. I have LIII edge EXAFS data from monometallic carbon supported Pt, Re and bi-metallic PtRe nanoparticles (1-3nm). My objective to perform a first coordination shell fit, and i have been successful with the monometallic data and reference samples (Pt and Re foils and salts). Unfortunately success with PtRe bimetallics has been limited so far, and I can not get a good fit with reasonable delEo values. Did anyone work with platinum/rhenium before and can offer some helpful advice? The literature says that even though the backscattering properties of Pt and Re are similar (given their proximity on the periodic table), they should still be distinguishable with k0-weighted fitting. I was also if its possible to use Athena or Artemis to extract the back-scattering properties of Pt and Re from reference samples, and use these properties in fitting a Pt-Re path. Thank you very much for your help -Ed Kunkes Graduate Student University of Wisconsin-Madison Chemical Engineering

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Platinum and Rhenium EXAFS in Artemis
by Edward L. Kunkes 15 Apr '07

15 Apr '07

Hi everyone, Sorry for the formatting error in my last e-mail. I am new to EXAFS and have been working with Athena to process my data and trying to fit it with Artemis. I have LIII edge EXAFS data from monometallic carbon supported Pt, Re and bi-metallic PtRe nanoparticles (1-3nm). My objective to perform a first coordination shell fit, and i have been successful with the monometallic data and reference samples (Pt and Re foils and salts). Unfortunately success with PtRe bimetallics has been limited so far, and I can not get a good fit with reasonable delEo values. Did anyone work with platinum/rhenium before and can offer some helpful advice? The literature says that even though the backscattering properties of Pt and Re are similar (given their proximity on the periodic table), they should still be distinguishable with k0-weighted fitting. I was also if its possible to use Athena or Artemis to extract the back-scattering properties of Pt and Re from reference samples, and use these properties in fitting a Pt-Re path. Thank you very much for your help -Ed Kunkes Graduate Student University of Wisconsin-Madison Chemical Engineering

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max number of restraints
by Victor.Streltsov＠csiro.au 15 Apr '07

15 Apr '07

> Hi Victor, > > It should be > restraint_sum = restraint1 + restraint2 + restraint3 + ... > > That is, a restraint should be a value that you'd like to me made as > small as possible, but can take either sign. The restraints are > simply appended to the "vector to be minimized" in the least squares > sense. > > I'm not sure that will fully solve the issue of having different > results between 10+ individual restraints and fewer summed restraints. > But I'm interested to hear and see more.... >--Matt >Given that individual restraints might be of either sign, you might >consider adding them in quadrature and using the square root of that >as the restraint. As Matt said, the details matter, but I think that >would be a serviceable work around for more than 10 restraints. >B I am confused. I thought I did exactly what Bruce said. I added them in quadrature and used square root of that as the restraint in artemis: After summing in quadrature: restraint_sum = sqrt((restraint1)^2 + (restraint2)^2 + (restraint3)^2...) I asked artemis "to build restraint from "restraint_sum"" and artemis added line: res_restraint_sum=penalty(restraint_sum,n1,n2)*scale This way of summing them seems closer to the way of adding them individually squared to the "vector to be minimized", provided artemis/ifeffit squares them before appending to the "vector" (you don't need to square them before). I guess the questions is what artemis does when asked to "build restraint from ..."? Victor Dr Victor Streltsov CSIRO Molecular & Health Technologies 343 Royal Parade Parkville VIC 3052 Australia Phone: +61 3 96627311 Fax: +61 3 96627101 victor.streltsov(a)csiro.au

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max number of restraints
by Victor.Streltsov＠csiro.au 13 Apr '07

13 Apr '07

Hi Matt, Thanks for your reply. Following Bruce's instructions, I have re-compiled Ifeffit with a larger "max_restraint" number on a Linux Machine. It works fine. Now, I can compare fits obtained with a set of multiple restraints (more elegant) and with a combined (limited to 10) set of restraints. I did combine restraints before in the way almost as you suggested. Assuming that each restraint is squared before adding to chi-square, I used sum as follows restraint_sum = sqrt((restraint1)^2 + (restraint2)^2 + (restraint3)^2...) for group of similar atoms and bond lengths. I guess, the difference in sums should not have a significant effect on fit. However, the difference in fits with multiple and combined restraints is surprisingly noticeable. I expected that the mathematical distinction should be pretty small as you said. But it is not at least in this test. I should explore it a bit more. The restraints combined were for similar sets of both lengths in independent molecular units and should not be strongly anti-correlated. I wonder if when stereochemical restraint relations are used and a large number of independent not equally important (close to zero elements) restraints imposed, convergence and stability of minimization algorithm are affected by such a sparse normal-equations matrix. Regarding the examples with possible more than 10 restraints, I have a data for a solution of Cu(II)complex with two almost geometrically identical organic molecules. The crystal structure and total 28 bond distances with uncertainties are available in the literature. This amount can be reduced to 14 assuming 2-fold symmetry between molecules. I wanted to apply in XAFS fit bit more rigorous independent restraints using available from crystal structure bond distances with uncertainties. In addition, for proper sterochemical restraints (e.g. in "rigid-body" refinement) some bond angles and planar groups restraints may be required, which may be important for multiple scattering paths. I guess, to have a bit more flexibility in the amount of restraints (more than 10) can be useful when fitting data for bio-molecular systems. Victor Dr Victor Streltsov CSIRO Molecular & Health Technologies 343 Royal Parade Parkville VIC 3052 Australia Phone: +61 3 96627311 Fax: +61 3 96627101 victor.streltsov(a)csiro.au >Hi Victor, >As Bruce said, the limit of 10 restraints is compiled-in. It could >certainly be increased. >But I'm impressed that you have more than 10 independent restraints to >impose! Depending on the nature of your restraints, it might be >possible to combine some of them. That is, using one restraint of > restraint_sum = (restraint1) + (restraint2) + (restraint3) >(where each restraint might be an elaborate calculation) is only >different from using three restraints if the three restraints are >strongly anti-correlated, so that they compete against one another. >If you're restraining different bond lengths, some of them probably >can be combined. >It's definitely better, and more elegant, to use multiple restraints, >but I think the mathematical distinction may be pretty small. >--Matt

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multiple k-weights refinments
by marie-helene.tuilier＠uha.fr 13 Apr '07

13 Apr '07

I try multiple k-weights refinments with ARTEMIS of Al K EXAFS of ternary aluminium and titanium nitrides films. This system is rather disordered and textured with large grain boundaries that contribute to EXAFS. Consequently only few parameters can be varied simultaneously. I think that the k**3 weighting plays the major role in the multiple k-weights calculation. There is no great dfference between the results of k**3 weight and multiple k-weights fits. Best regards Marie-Hélène Tuilier Laboratoire MMPF (Mécanique, matériaux et Procédés de Fabrication) Pôle STIC-SPI-Maths, Université de Haute Alsace, IUT de Mulhouse, 61 rue Albert Camus, F-68093 Mulhouse Cedex Tel : 33 (0)3 89 33 75 16 Fax : 33 (0)3 89 33 75 05

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my last message....
by Bruce Ravel 11 Apr '07

11 Apr '07

Sigh.... If any of you had any lingering doubts about what kind of an idiot I am, my last message -- mistakenly sent here rather than to the individual in question -- should clear that right up! Sorry for filling up your mail boxes.... B -- Bruce Ravel ---------------------------------------------- bravel(a)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/

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Mehmet Alper Sahiner/FAC/SHU is out of the office.
by Mehmet Sahiner 11 Apr '07

11 Apr '07

I will be out of the office starting 04/09/2007 and will not return until 04/15/2007. I will be at the MRS2007 Meeting in San Francisco between 4/9/07-4/15/07 Alper

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max number of restraints
by Victor.Streltsov＠csiro.au 11 Apr '07

11 Apr '07

Hi, I wonder if there is a way to change a max number of restraints in Artemis or Ifeffit. If you have more than 10 restraints in Artemis (which is possible to input in Artemis without a warning), the Ifeffit gives the following message in "ifeffit interaction buffer": *** feffit:: too many restraints restraint will be ignored: and ignores all restraints after the 10th in the Artemis list. Often, you need more than 10 restraints when fitting data for biological samples (e.g. bond length restraints etc.) Thanks Victor

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setting root for artemis and athena
by Katy Evans 11 Apr '07

11 Apr '07

This may be really simple but I haven't managed to work out how to do it or find it in the instructions. Is there a way to tell Artemis or Athena where to start when you go to open or save a file? If I were to add up the time I spend clicking up through the directories then I suspect I would discover that it would be sufficient for me to learn to play the piano or similar. Maybe. Cheers, Katy katy.evans(a)anu.edu.au RSES, ANU, Mills Rd, ACT 0200 Ph: 02 61254228 Fx: 02 61258253

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Help with MoOx fitting
by g d 05 Apr '07

05 Apr '07

Hi all, Is there anyone having experience with MoOx fitting? The sample I have is MoOx supported on structured silica (most possibly with tetrahedral coordination, because it's R space and EXAFS is very like that of (NH4)2MoO4). The only standard atom.inp files I have are MoO3.inp and MoO2.inp. I didn't get good fitting with these two. Is it better that I use the standard file that corresponding to tetrahedron? If so, could any one share the atom.inp file with me? Thank you so much! Gd _________________________________________________________________ The average US Credit Score is 675. The cost to see yours: $0 by Experian. http://www.freecreditreport.com/pm/default.aspx?sc=660600&bcd=EMAILFOOTERAV…

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DWF's for amorphous InP
by Claudia Schnohr 05 Apr '07

05 Apr '07

Hello everyone. I am a PhD student and I have encountered a problem with analysing the EXAFS of amorphous InP. For amorphous InP the first shell around an In atom is comprised of both P and In atoms. The In leads to a small peak in the R-spectrum that strongly overlaps with the bigger peak due to scattering from P. If I use two different Debye-Waller-factors, one for each scatterer, and let them both float during the fit I get weird values since the coordination numbers for both peaks have to be floated as well. Therefore, some restraint is needed for the DWF's. Is there any correlation between the two DWF's following from theory or experiment that I could use to restrain my fitting parameters ? Are there other possibilities to handle such a situation ? Many thanks in advance for your help, Claudia -- Claudia S. Schnohr Department of Electronic Materials Engineering Research School of Physical Sciences and Engineering The Australian National University Canberra, ACT 0200 AUSTRALIA

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Correlation between SO2 and sigma^2
by Ricardo Faccio 30 Mar '07

30 Mar '07

Hi Bruce and Scott for your valuable comments I am performing refinenments over a serie of cuprates. In this case La2CuO4 should work as a theoretical standard. This is why I wrote to you, but my "interesting" samples don't have much problems. The theoretical standard is the problem! quite crazy! Thanks again I will try Bruce's comments. Best regards Ricardo ------------------------------------------------------------------------- ----- Ing. Quím. Ricardo Faccio Mail: Cryssmat-Lab., Cátedra de Física, DETEMA Facultad de Química, Universidad de la República Av. Gral. Flores 2124, C.C. 1157 C.P. 11800, Montevideo, Uruguay. E-mail: rfaccio(a)fq.edu.uy Phone: 598 2 9241860 Int. 109 598 2 9290705 Fax: 598 2 9241906 Web: http://cryssmat.fq.edu.uy/ricardo/ricardo.htm

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Fitting EXAFS with out any atomic coordinates?
by Mario Gomez 30 Mar '07

30 Mar '07

Hi Bruce my name is Mario and I was just wondering if your program can fir the EXAFS spectra without any atomic positions. Obviously the answer is no if you use atoms to create the FEFF file but what does one do when we have a semi-crystalline material for which no one has attempted the fractional coordinates. I mean the XRD pattern shows some crystalline but due to the high background I would think it would be very hard to do the Rietveld analysis. So what can one do, can we fit the EXAFS spectra knowing only the chemical composition and perhaps the lattice geometry(orthorhombic or hexagonal)? thanks

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Re: [Ifeffit] Ifeffit Digest, Vol 49, Issue 20
by Ricardo Faccio 29 Mar '07

29 Mar '07

Dear Iffefit users I am performing a refinement for a La2CuO4 in powder. I followed Bruce's course, and I tried determine S02 hopping its correlation with sigma^2. I am attaching a graph of this determination. Please see enclosed the file. My question is: Is common to obtain a S02 of 0.625? and is common to obtain a nearly sigma^2=0? I didn't fit E0, alpha and degeneracy for the first single shell. What is the best strategy to fix this bad result? Thanks in advance Ricardo ------------------------------------------------------------------------- ----- Ing. Quím. Ricardo Faccio Mail: Cryssmat-Lab., Cátedra de Física, DETEMA Facultad de Química, Universidad de la República Av. Gral. Flores 2124, C.C. 1157 C.P. 11800, Montevideo, Uruguay. E-mail: rfaccio(a)fq.edu.uy Phone: 598 2 9241860 Int. 109 598 2 9290705 Fax: 598 2 9241906 Web: http://cryssmat.fq.edu.uy/ricardo/ricardo.htm

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Re: [Ifeffit] problem with running feff while working your ZnO example file
by Scott Calvin 29 Mar '07

29 Mar '07

Hi Soma, I am cc'ing this to the ifeffit list, as they may have a better answer than I do. The ZnO tutorial does currently have some problems having to do with variable names that became reserved after it was written. Those problems, though, do not cause the symptom you describe. The output of your feff run is "strange"--it claims to have an Rmax of 0, for instance. I'm not sure what could cause that particular problem; hopefully someone else on the list could help you. To start with, information on your operating system, version of Artemis, etc. might be helpful. --Scott Calvin Sarah Lawrence College At 06:29 PM 3/28/2007, soma(a)agni.phys.iit.edu wrote: >Hi Scott > >I work as a beamline scientist at MRCAT, sector 10 in APS where Bruce and >Shelly do their expts and I am trying to learn EXAFS analysis. Bruce >showed me your worked out example on ZnO and I was trying to follow it. > >I followed your notes till where it says to click the button "Run Feff". >When I do that ARtemis palatte gives the following message with red error >message at the end. I have copied that below. When I try putting RMAx 5 >instead of 6, it still keeps on giving the same message. i checked that >nleg was 4 and did not change it. > >I closed all programs in my linux based computer but message keeps on >appearing. Could you please let me know what I should do to proceed. > >Thank you. >Soma > >Soma Chattopadhyay >Beamline Scientist >MRCAT Sector 10 Building 433B >Advanced Photon Source >Argonne National Laboratory >9700 South Cass Avenue, Argonne >IL 60439, USA. >Phone: 1-630 252 0348 >FAX: 1-630 252 0358 >Email: soma(a)agni.phys.iit.edu > >************************************************************************** > >Scott Calvin's ZnO project > Calculating potentials and phases... > free atom potential and density for atom type 0 > free atom potential and density for atom type 1 > free atom potential and density for atom type 2 > overlapped potential and density for unique potential 0 > overlapped potential and density for unique potential 1 > overlapped potential and density for unique potential 2 > muffin tin radii and interstitial parameters > phase shifts for unique potential 0 > phase shifts for unique potential 1 > phase shifts for unique potential 2 > Preparing plane wave scattering amplitudes... > nncrit in prcrit 9 > Searching for paths... > Rmax 0.0000 keep and heap limits 0.0000000 0.0000000 > Preparing neighbor table > nfound nheap nheapx nsc r > Internal path finder limit exceeded -- path list may be incomplete. > Paths found 0 (nheapx, nbx 1 0) > Eliminating path degeneracies... > Plane wave chi amplitude filter 2.31% > > > Feff's path finder warned of reaching a memory limitation which > caused it not to compute the complete list of paths for your > cluster. There are two ways to avoid this problem when running > Feff. One is to decrease the size of the RMAX parameter in the > Feff input data. The other is to have the Feff calculation not > consider high order multiple scattering paths. Putting "NLEG 4" > in the Feff input data will limit Feff to compute only up to > four-legged (triple scattering) paths. > >**********************************************************************

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Re: [Ifeffit] Fitting EXAFS with out any atomic coordinates?
by Scott Calvin 28 Mar '07

28 Mar '07

Hi Mario, There are a couple of different ways to approach the kind of problem you describe, although you must be willing to guess at what the local environment looks like. For example, maybe the chemical composition tells you that you have an iron oxide. At a minimum, you could use Atoms to make a crystal structure for, say, hematite, and then only fit the nearest-neighbor paths, perhaps with an unknown coordination number. This can give you some information. Or, following the same example, maybe you think your material might be like a disordered hematite. Then you could try to fit the hematite structure out for several shells, but with free parameters to account for disorder (e.g. you could include vacancies). The key questions that you should not lose track of: --What would you like to know about your material? --What do you think your material might be like? I think sometimes people forget in particular about the first question. EXAFS doesn't answer generic questions very well ("tell me all about my sample"), but it often answers particular questions quite effectively ("are my iron atoms bonded to oxygen, or sulfur?"). So if you know what your particular questions are you've got a good chance of getting help using Bruce's and Matt's software. --Scott Calvin Sarah Lawrence College At 02:15 PM 3/28/2007, you wrote: >Content-class: urn:content-classes:message >Content-Type: multipart/alternative; > boundary="----_=_NextPart_001_01C77165.0DE07268" > >Hi Bruce my name is Mario and I was just wondering if your program >can fir the EXAFS spectra without any atomic positions. Obviously >the answer is no if you use atoms to create the FEFF file but what >does one do when we have a semi-crystalline material for which no >one has attempted the fractional coordinates. > >I mean the XRD pattern shows some crystalline but due to the high >background I would think it would be very hard to do the Rietveld analysis. > >So what can one do, can we fit the EXAFS spectra knowing only the >chemical composition and perhaps the lattice geometry(orthorhombic >or hexagonal)? > > > >thanks >_______________________________________________ >Ifeffit mailing list >Ifeffit(a)millenia.cars.aps.anl.gov >http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

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[howto] configurational average in EXAFS with FEFF
by Mauro Rovezzi 28 Mar '07

28 Mar '07

Dear all, I would like your point of view on how to approach configurational average in EXAFS analysis with FEFF (let's not consider GNXAS for the moment). In particular, I'm trying to combine ab-initio DFT numerical simulations with EXAFS for doped semiconductors where in the minimized cluster there are N absorbing atoms. Usually I don't use the CFAVERAGE card because fractional degeneracies are not taken into account and, to have a qualitative analysis, I calculate an "extended"-XANES (up to 8.0 A^-1) for each cluster - where each absorbing atom is shifted to (0,0,0) - and then I average over all single-cluster signals. Well, at this point my question is: how to proceed in the quantitative analysis with Ifeffit? In fact, considering only single-scattering expansion for all the clusters the result could be seen as a radial distribution for each scattering pair where the integral gives the coordination number and the FWHM a structural Debye-Waller factor. How to combine these informations? Thanks for the answers, Mauro -- Mauro Rovezzi <rovezzi(a)esrf.fr> PhD student - GILDA BM08 c/o ESRF (Grenoble, FR)

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the ifeffit wiki
by Bruce Ravel 26 Mar '07

26 Mar '07

Hi everyone, I just finished cleaning some really distasteful vandalism off of the Ifefift wiki. Several pages had been edited to add dozens of links to pages that have .... ummmm .... very little to do with XAS. Such vandalism is unwelcome on the ifeffit wiki and is most certainly not what Matt and I intended when we started it. I check for this sort of thing fairly often, but much of the latest crop of vandalism had been there for 24 hours without my noticing. If you ever see content on a wiki page that is clearly the work of a wiki vandal, feel free to clean it up or to drop me a private email and I will do so. In any event, please do not be mad at those of us who administer the wiki. We really intend the ifeffit wiki to be a place where you can bring your children to learn about XAS ;-) B -- Bruce Ravel ---------------------------------------------- bravel(a)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/

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