Re: [Ifeffit] What does FEFF stand for?
On 5/10/2011 2:49 PM, Francisco Garcia wrote: Dear all, I wish to ask a somewhat novice question: What does the acronym FEFF stand for? Thank you. ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit I am pretty sure it stand for the calculated Effective Scattering factor F(*eff*)ective. buena salud, Chris Patridge ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] What does FEFF stand for?
On Tuesday, May 10, 2011 03:03:23 pm Scott Calvin wrote: My understanding, although I could be wrong is that the effective part came from an improvement of the theory to account for curved-wave effects. In other words, early theories approximated the photoelectron as a plane wave, but of course it spreads out radially from the absorbing atom. That change necessitated tweaking the definitions of the factors, so it became the effective f. I think you are mistaken. My memory of the etymology has to do with the formalism dating back to Feff5 for computing MS paths. For a purely single scattering theory, you have an F and a phi (without the subscript eff). That is, you can simply compute the scatting function for the one scatterer and be done with it. Feff's path expansion introduced two clever things to the EXAFS business. One is that it provided a formalism for computing a single function that takes into account the angle-dependent scattering functions of all atoms in an arbitrary-geometry multiple scattering path. This allows one to treat a MS path with the familiar SS EXAFS equation only by replacing F and phi with F_eff and phi_eff. That innovation is central to how Ifeffit works. The second clever thing is that it's really fast. That's not such a big deal today, but back in the mid-90s, when a Feff run could take several minutes, a faster algorithm was very welcome indeed. B -- Bruce Ravel bra...@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage:http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/ ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] What does FEFF stand for?
You are both mistaken, but do not check what FEFF is on Urban Dictionary, as it is inappropriate. Anatoly From: ifeffit-boun...@millenia.cars.aps.anl.gov on behalf of Ravel, Bruce Sent: Tue 5/10/2011 3:16 PM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] What does FEFF stand for? On Tuesday, May 10, 2011 03:03:23 pm Scott Calvin wrote: My understanding, although I could be wrong is that the effective part came from an improvement of the theory to account for curved-wave effects. In other words, early theories approximated the photoelectron as a plane wave, but of course it spreads out radially from the absorbing atom. That change necessitated tweaking the definitions of the factors, so it became the effective f. I think you are mistaken. My memory of the etymology has to do with the formalism dating back to Feff5 for computing MS paths. For a purely single scattering theory, you have an F and a phi (without the subscript eff). That is, you can simply compute the scatting function for the one scatterer and be done with it. Feff's path expansion introduced two clever things to the EXAFS business. One is that it provided a formalism for computing a single function that takes into account the angle-dependent scattering functions of all atoms in an arbitrary-geometry multiple scattering path. This allows one to treat a MS path with the familiar SS EXAFS equation only by replacing F and phi with F_eff and phi_eff. That innovation is central to how Ifeffit works. The second clever thing is that it's really fast. That's not such a big deal today, but back in the mid-90s, when a Feff run could take several minutes, a faster algorithm was very welcome indeed. B -- Bruce Ravel bra...@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage:http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/ ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit winmail.dat___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] What does FEFF stand for?
I think Scott is right that the original meaning of effective was that the scattering amplitude is not for point scattering of a plane wave, as was used in earlier work (say, Sayers, et al 1971). Feff3 (circa 1990) didn't to do multiple scattering, but did put in curved wave effects. http://leonardo.phys.washington.edu/feff/Docs/feff3.html But once you move away from the point scattering of plane waves, effective can fold everything else in. So now effective does include mapping the MS paths to a SS formalism does make more effective (less native point-scattering of plane waves). The fast, clever matrix representation for MS paths of Rehr and Albers was incredibly important for making the calculations actually useful. I believe others (Schaich? Gurman?) were calculating MS on triangles and 4-legged focused paths somewhat before Rehr, but that it was slow and hard to generalize. Feff5 generalized this for all orders. --Matt ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
Re: [Ifeffit] What does FEFF stand for?
Everyone is at least partially right. In our PR B34,4350(86) paper (which was rejected by PRL) we noted that the exact single scattering XAFS eq. could be recast in the same form as that of Sayers Stern Lytle (PRL 27,1204(71) by replacing the backscattering amplitude f(pi) with the exact curved wave expression given by ~f (see our Eq. 9). For single scattering this ~f is equivalent to the Z factor in Lee and Pendry and a similar quantity in Schaich. We replaced ~f with f_eff (i.e., the effective scattering amplitude)in our PRB 44, 4146 (91), which also describes the single-scattering FEFF code. This code started in pieces at Los Alamos in 1984, but the integrated version with a reliable overlapped atom potential became FEFF3 some years later. The name FEFF originated during a meeting with Jose Mustre de Leon, Steve Zabinsky and others (?) when we were looking for a catchy name for the code. Although it was not an acronym, I thought FEFF seemed quite appropriate. Interestingly, we found that one can also define an exact effective scattering amplitude for multiple-scattering paths, e.g. as in PRL 69, 3397 (92). The beauty of this representation is that permits an interpretation of both single- and multiple-scattering contributions using the standard EXAFS equation. Thus in subsequent papers and code developments we simply used f_eff to describe all SS and MS paths. Thus the name FEFF was also retained for subsequent versions of the code. J. Rehr On Tue, 10 May 2011, Francisco Garcia wrote: Dear all, I wish to ask a somewhat novice question: What does the acronym FEFF stand for? Thank you. ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit ___ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
