Jacob, Actually the R factors including the Bijvoet pairs would be higher, because the uncertainties in F(+) and F(-) are higher than that of F(mean) by a factor of about sqrt(2). R factors will always be higher for unmerged data because averaging always reduces the uncertainty. This means that we are in effect 'cheating' by throwing away the relatively imprecise anomalous differences and getting falsely lower R factors as a result! But as you imply the model would improve if we included the anomalous data in the refinement (assuming of course that it is meaningful data). This just demonstrates that low R factors do not necessarily equate to a better model - especially if you deliberately throw away the less precise data! The model would improve (marginally) because the anomalous differences would obviously provide additional information about the anomalous scatterers and therefore increase their precision, but wouldn't affect the precision of the lighter atoms. But is imprecision in the parameters of the heavy (or heavier) atoms usually an issue? - since these have bigger real scattering factors they will be more precisely determined than the lighter atoms anyway. So I don't think you would gain very much, except maybe more truthful R factors!
Cheers -- Ian On 10 January 2012 20:00, Jacob Keller <j-kell...@fsm.northwestern.edu> wrote: > Dear Crystallographers, > > it seems to me that on a certain level we are always throwing away > (sort of) about half of our data when we merge Bijvoet pairs--why > shouldn't we keep them separate, since we know that they should be a > little bit different, especially in light of the higher multiplicities > which are more common now? Shouldn't modelling them give better > R-values, and wouldn't it just be more true? I guess a sort of proof > for this is that sulfurs are almost always detectable on anomalous > difference maps, implying that we are actually measuring those > differences accurately enough to see them (I don't think these things > can arise from model bias, as anomalous differences are not modeled.) > At least maybe at the final steps of refinement...? > > JPK > > -- > ******************************************* > Jacob Pearson Keller > Northwestern University > Medical Scientist Training Program > email: j-kell...@northwestern.edu > *******************************************
