Hi Chris, There is nothing sacred about Rbkg=1.0 Ang. It was chosen as a reasonable default. If you know you have atoms much closer than 2Ang, lowering Rbkg to half the first neighbor distance is a fine place to start. You may have to play with this value (and the k-weight used in the background) a bit more than otherwise, but it should work out.
Scott wrote: > ... I'll admit that the "fit background" button confuses me a bit, > though (I always have to spend ten minutes convincing myself again as to > what exactly it is doing), so someone else should explain how that can be > used to help address your question. In Artemis, when you check "refine the background", the following things happen: 1. Your R-space fitting range [Rmin,Rmax] becomes [0,Rmax] 2. A spline is added to the model chi(k): model = spline(k) + sum_of_paths(chi(k)) 3. The spline is chosen with breakpoints equally spaced in k, using Rmin as Rbkg to select the number of breakpoints. --Matt On Fri, Oct 23, 2009 at 3:24 PM, Scott Calvin <scal...@slc.edu> wrote: > Hi Chris, > One thing to keep in mind is that it is not wrong to eliminate part of your > signal; it just means you're losing a little bit of data. This is similar to > a common filtering mistake that beginners make when trying to choose the > maximum end of their fitting range: they look at the paths they are > including, and try to set Rmax high enough to include most of the > contribution from the paths they are including. What they should be doing, > however, is to look at the paths they are not including, and set Rmax low > enough so that the contributions from those paths are tolerably small. > Looking at correlations between background parameters and fitted parameters > when the "fit background" option is selected also helps provide you with > information. I > --Scott Calvin > Sarah Lawrence College > On Oct 23, 2009, at 3:51 PM, Chris Patridge wrote: > > Hello everyone, > > In removing background, most literature suggests Rbkg value of 1.0 because > below this represents mostly noise and low freq components not part of the > scattering effect. In using Feff and viewing the individual paths > calculated a number of them have paths near 1.5 A, therefore meaning that > they have some contributions very close to 1.0 A and below in R space due to > phase shift. Has anyone modeled materials which contain these rather short > Reff and how did you decide what was scattering and noise? > > Thank you all, > > > > _______________________________________________ > 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