Hello Mike, Thank you so much for your detailed explanation. I do appreciate your help. I agree with you that walking through these steps with an expert will be much helpful. I should attend some XAFS workshop in the future. Thanks again for your help.
Have a great weekend! Best, Yunyun On Thu, Oct 26, 2017 at 6:17 PM, Mike Massey <mmas...@gmail.com> wrote: > The way I usually do it is to just pick one of the FEFF paths (instead of > both) corresponding to the distance you care about. I actually haven't used > the latest version of Artemis to do any fitting recently, but I assume it > is still similar. > > In your list of FEFF paths, pick C1.1, right-click and "include C1.1 in > the fit." Then you define your fitting parameters for that atom at that > distance, one of which should be the coordination number of the atom (I > typically might define amplitude as S02*CN and put path degeneracy at 1, > but there are probably other, better ways to do this — I hope someone more > knowledgeable might correct me if this is a poor approach). > > So then your amplitude fitting parameter, "amp_C1," basically becomes a > proxy for the coordination number of that atom at that distance, and > "ss_C1" is your measure of disorder. So instead of trying to fit two C at > 1.94 and 1.99 Angstroms individually, you model that with a single path of > C at around 1.97 Angstroms (or whatever FEFF's R plus your fit's delr is) > with disorder ss_C1, and you either set the coordination number to 2, or > you guess it in the fit and hope it comes out to around 2, since it > probably should. > > There are probably a lot of ways to approach this, every person seems to > have their preferred method. You might consider setting up a screen-sharing > videoconference with someone and get them to walk you through their process > for fitting. It seems to be something that is effectively learned through > apprenticeship. Another possibility is attending an EXAFS fitting workshop, > if any are coming up. > > I recommend getting a really good handle on single-phase fitting before > even attempting multi-phase fits. If appropriate, you can even just do LCF > for the multi-phase system. > > > Just my two cents, hope my thoughts are helpful and not grossly wrong (if > wrong, hopefully someone will correct me — thanks in advance). > > Cheers, > > > Mike > > > > > > On Oct 26, 2017, at 2:06 PM, Yunyun Zhou <yyzhouz...@gmail.com> wrote: > > Hello Mike, > > Thank you very much for your help. I have a question about combining > different atoms in a shell. For example, in the Fe5C2 model, > ATOMS * this list contains 221 atoms > * x y z ipot tag distance > 0.00000 0.00000 0.00000 0 Fe1 0.00000 > -0.19385 1.76888 -0.78107 4 C1.1 1.94335 > -0.19385 -0.94552 1.72493 4 C1.2 1.97661 > > If I am going to combine C1.1 and C1.2, shall I set them the cartisian > coordinate as in FEFF file? > > ATOMS * this list contains 221 atoms > * x y z ipot tag distance > 0.00000 0.00000 0.00000 0 Fe1 0.00000 > 0.00000 0.00000 1.94335 4 C1.1 1.94335 > > Thanks a lot for your help again! > > Best, > > Yunyun > > > On Thu, Oct 26, 2017 at 4:40 PM, Mike Massey <mmas...@gmail.com> wrote: > >> I definitely recommend combining multiple atoms of similar distances into >> the same path. That's what you end up with anyway if you use multiple >> paths, with a lot more potential problems in the software. >> >> Sometimes it is beneficial to split a shell of of atoms into two, to >> capture some physical behavior (for example, 2 atoms at 3 Angstroms and 4 >> atoms at 3.3 Angstroms, instead of one very disordered shell of 6 atoms at >> 3.2 Angstroms). >> >> Depending on your data quality, I recommend building a fitting model with >> maybe 2-8 total "shells" of atoms at different distances. >> >> >> >> Mike >> >> >> > On Oct 26, 2017, at 12:52 PM, Yunyun Zhou <yyzhouz...@gmail.com> wrote: >> > >> > Hello All, >> > >> > I am having trouble fitting my sample with both Fe5C2 and Fe3O4 as >> standards. Hopefully somebody could help me out and I really appreciate it. >> > >> > My iron sample was mainly iron carbide in the core and iron oxide on >> the shell. Fe5C2 has a monoclinic structure and generated complicated FEFF >> paths. Many of the paths have similar distances. I have found some paper >> reported their results as follows, which apparently combined several paths >> together and assume they have similar path length. My first question is >> does someone know how they did it? It seems that combing similar paths >> makes fitting much easier, but I am not sure whether this is a correct way >> to do the fitting. >> > >> > My second question is regarding fitting with the two combined models of >> Fe5C2 and Fe3O4. I haven't fit with two models before. Is it just like what >> I did in the attached Artemis file? Shall there be any restrictions between >> the two models and do I have to fix some of the parameters? >> > >> > Thank you very much in advance for your help! >> > >> > Best, >> > >> > Yunyun Zhou >> > >> > Fe5C2 FEff paths: >> > <image.png> >> > >> > Literature reported results: >> > <image.png> >> > <artemis_CWV 130 spent_example.fpj> >> > _______________________________________________ >> > Ifeffit mailing list >> > Ifeffit@millenia.cars.aps.anl.gov >> > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >> >> _______________________________________________ >> Ifeffit mailing list >> Ifeffit@millenia.cars.aps.anl.gov >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit >> Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit >> > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit > >
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