[Ifeffit] Number of independent points: Artemis & Feffit

[Bruce Ravel]

On Tuesday 28 March 2006 04:50, Mauro Rovezzi wrote:
> Hi guys,
>
> I've noticed a strange behaviour in Artemis during the calculation of
> N_idp; the value differs from the Feffit one. In fact, with command line
> feffit (ver. 2.984) N_idp=(2*delta_k*delta_R/pi)+2 but with Artemis the
> "+2" value disappears. Seen that the procedure is:
> Artemis-->Ifeffit-->feffit(), why they differs?

Mauro,

Artemis needs to be able to report on Nidp at any time, even before
any commands have been sent to Ifeffit.  Consequently, it computes
Nidp by itself based on the current state of the project.

You are correct that it reports N_idp=(2*delta_k*delta_R/pi) -- I just
looked at the code and that is indeed what it does.  I don't think,
however, that you are correct that Ifeffit adds the 2.  I also looked
at the Ifeffit code.  At line 341 in iff_feffit.f, Nidp is computed
without the added constant.  (But perhaps I am missing something else
in the code...?)

I just ran a fit with Artemis -- Ifeffit's value of n_idp in that fit
was 18.56 and Artemis reported 19.

It is true that there is a difference in how Ifeffit computes Nidp
during a fit and how Artemis computes it when asked.  Ifeffit uses
that actual grid values of the first and last points of the actual
data grid in the Fourier transform range in k and of the fit range in
R.  Artemis uses the numbers entered into the k-range and R-range
boxes on the data page.  It is not unbelievable that those values
might conspire to be different by 1.

B

-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

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