[Ifeffit] Query regarding measurement uncertainity

Matt Newville newville at cars.uchicago.edu
Mon Jan 26 10:09:50 CST 2009

Hi Bindu,

On Mon, Jan 26, 2009 at 8:28 AM, Bindu R. <binduiuc at yahoo.co.in> wrote:
> I have one doubt regarding the measurement uncertainty in R- space (εR). In
> the manual it is written that εR is evaluated from the rms value of FT in
> the range 15-20 Å. My doubt is.....
> In case the FT in R-space does not extend up to the range mentioned above,
> then how does the fitting program choose the value of εR?

The FT transform from k to  R space that ifeffit does always extends
to R = 10*pi Ang.  The R-range saved into a data array is usually much
shorter (it's usually out to 10Ang), though you can change this if
you'd like.   There are always high-frequency components of a real
XAFS spectra.

Anyway, when doing a fit, the data is FT to R-space internally, and
the range between 15 and 20 Ang is used to estimate epsilon_R.
Knowing the FT parameters (especially the k-weight), that can be
converted into epsilon_k.


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