On Friday 18 January 2008 07:50, Santarsiero, Bernard D. wrote: > I do recall Rmerge being more popular with the small molecule > crystallographers. However, I also recall a difference between averaging > over pairs of reflections that were or were not Bijvoet pairs, for even > small differences in the anomalous scattering contributions.
That matches my understanding. Rmerge is an average over replicate measurements of the intensity for identical [hkl]. Rsym is an average over the measurements for all symmetry equivalent reflections. In the presence of anomalous scattering, Rsym will be higher than Rmerge because the Bijvoet pairs, although symmetry related, do not have identical intensities. One might logically report two values for Rsym, one which averages over the Bijvoet-paired reflections and one which does not. Although the data processing programs are happy to make this distinction (e.g. the "Anomalous" and "Scale Anomalous" tick-boxes in HKL2000), the separate values obtained are rarely seen in a published "Table 1". > I seem to recall hearing Rsym first when I used the Xuong-Hamlin detector. Same for me. Before that, as a neophyte graduate student, I used a diffractometer and foolishly programmed it to collect only the minimal set of data corresponding to one asymmetric unit of reciprocal space. This demonstrated my understanding of space group symmetry, but also my ignorance of statistics and good experimental practice. Since the resulting data sets contained no symmetry equivalents, there was no Rsym to be calculated. There was still an Rmerge if multiple crystals were used for the collection, however. -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle 98195-7742
