>Using the LHPM/REIT7/SR5 software, occassionly when refining subtle
>parameters such as site occupancies or thermal parameters, the R-Wp can
>go down while the R-Bragg goes up. Are then any comments on this as it
>implies at this level - it would be better to be refining and minimising
>on the weighted R-Bragg, not the weighted profile(?).
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