[Ifeffit] Re: I should be able to find this somewhere...

[Bruce Ravel]

Hi,

In fact, Artemis and Athena both attempt to deal correctly with input
chi(k) data that is not on the expected grid.  Both codes perform a
test to see if the data is on the standard grid and, if not,
front-loads the data with zeros and uses Ifeffit's boxcar rebinning
function to put the data on the expected grid.

Alas, there was a bug in the rebinning macro used by A&A which caused
it to fail in a particular situation met by the data in Matthew's
example.  This bug will be fixed in the next release.

If you import mu(E) data into Athena and then import data from the
Athena project file into Artemis, you will never see the problem that
Matthew reported.

B


On Tuesday 17 January 2006 16:09, Matt Newville wrote:
> Hi Matthew, Bruce,
>
> OK, I think it's a bug.  Well, maybe it's just a consequence of a
> "known but not well advertised" problem.
>
> The chi(k) data you have doesn't start at k=0.  By itself, this should
> be OK, and the fit seems to be mostly fine.  There seems to be a bug
> in Ifeffit that the output fit chi(k) has the same number of data
> points as the input chi(k) data. In this case is incorrect -- it
> should go to the last data point in k.  From looking at the code, I'm
> pretty sure this has no effect on the fit and is only an error in the
> output array.  But the shortened chi_fit(k) will have an impact on the
> way Bruce is calculating the residual.   Even if that were fixed, the
> residual would still be wrong, as the residual in Artemis is
> calculated as
>    set(data0_res.k = data0.k)
>    set(data0_res.chi = data0.chi - data0_fit.chi)
>
> and then Fourier transforming to R-space.   Since the data and fit
> aren't aligned in k-space, the residual is incorrect.  It should be
>
>   set(data0_res.chi = data0.chi - interp(data0_fit.k, data0_fit.chi,
> data0.k))
>
> or some variation on that.
>
> Sorry for the trouble.  It's definitely a result of narrow-mindedly
> expecting that all chi(k) data comes from Ifeffit/Athena and will
> always start at k=0.  We should get this fixed (it's mostly there!) so
> that data from other sources can be used.  Right now, the easiest
> work-around is to pre-pack the chi(k) data with zeros (starting from
> k=0.00)
>
> --Matt
>
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-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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