Guys, This appeared in a Rietveld e-mail a bit ago & needs a comment: "While I can understand the general rationale for the idea (minimize the weight of the very strong reflections to the final integrated intensity for the reflection)"
The fact of the matter is that most least squares programs doing Rietveld refinement (GSAS included) use weights that are equal to 1/I. This is what one expects from "pure" Poisson counting statistics for reasonably large numbers of counts (i.e. >20). Because of this choice of weights, each observation in the refinement is "equal" in terms of "impact" on the refinement. So the above suggestion that the weights for strong reflections be "reduced" is already done in the standard form of Rietveld least squares refinement. The "extra" residual you may see in the vicinity of strong reflections is actually no larger when weighted (unless the model isn't right, of course) than the surrounding lower intensity values. In GSAS, the graphics routine POWPLOT has an option that clearly illustrates this. The "W" option scales each intensity & difference by the weight. The resulting curve has no peaks(!) and is what the LS minimization engine actually sees for refinement. Bob Von Dreele R.B. Von Dreele IPNS Division Argonne National Laboratory Argonne, IL 60439-4814
