Re: RIET: Refining on R-Bragg rather than R-Wp???

Armel Le Bail ( armel@fluo.univ-lemans.fr )
Thu, 11 Sep 1997 17:07:47 +0200

Lachlan wrote :

>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(?).

If minimizing Rwp, then Icalc and true Iobs are involved. Trying to
minimize R-Bragg, squared Fcalc but not true squared Fobs will be
available (only biased "|Fobs|" depending on Fcalc when there is
overlapping). Consequently I can't see how a refinement based on
minimising R-Bragg could be credible. You should minimize an
expression depending on the really observed data.

Armel