[Ifeffit] Rbkg value

[Matt Newville]

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 <SCalvin at 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,
>
>
>
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