Re: RIET: Refining on R-Bragg rather than R-Wp???
Dr Erich Kisi ( meehk@cc.newcastle.edu.au )
Fri, 12 Sep 1997 12:50:59 +1000
>Bob Von Dreele Wrote
>
>As others have said already, this is a somewhat confused question. As a
reminder, least
>squares refinement is about matching a set of observations to a comparable
set of computed
>values created from some mathematical model. Expressed in this way it is
obvious that the
>Bragg reflections are not observations in a powder diffraction pattern. The
observations are
>the suite of individual profile intensities obtained as a function of an
independent
>"scanning" variable (2-theta, TOF or whatever). The Bragg peaks, on the
other hand, are our
>method of modeling the peaky features of a powder diffraction pattern. They
have no
>independent existence as observations. Thus, minimization on Fo-Fc makes no
sense because
>there are no real Fo's. Those things that are obtained via some extraction
procedure during
>a Rietveld refinement are developed by assuming that the Fc's are in the
correct ratio; thus
>the "Fo's" are biased by the Fc ratios. And things like R(Bragg) are only
useful as probably
>biased diagnostic tools and, most assuredly, not real refinement residuals.
>Bob Von Dreele
Of course, a somewhat similar argument can be made with respect to single
crystal data ie they are assembled from a series of step intensities after
correction for geometric factors and after background (non-physical)
subtraction. They too are therefore not actual observations, however they
ARE independent of the model as many have pointed out.
Erich Kisi