It is instructive to look at what happens for small molecules where there is often no solvent to worry about. They are often refined using SHELXL, which does indeed print out the weighted R-value based on intensities (wR2), the conventional unweighted R-value R1 (based on F) and <sigmaI>/<I>, which it calls R(sigma). For well-behaved crystals R1 is in the range 1-5% and R(merge) (based on intensities) is in the range 3-9%. As you suggest, 0.5*R(sigma) could be regarded as the lower attainable limit for R1 and this is indeed the case in practice (the factor 0.5 approximately converts from I to F). Rpim gives similar results to R(sigma), both attempt to measure the precision of the MERGED data, which are what one is refining against.
George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582 On Wed, 27 Oct 2010, Ed Pozharski wrote: > On Tue, 2010-10-26 at 21:16 +0100, Frank von Delft wrote: > > the errors in our measurements apparently have no > > bearing whatsoever on the errors in our models > > This would mean there is no point trying to get better crystals, right? > Or am I also wrong to assume that the dataset with higher I/sigma in the > highest resolution shell will give me a better model? > > On a related point - why is Rmerge considered to be the limiting value > for the R? Isn't Rmerge a poorly defined measure itself that > deteriorates at least in some circumstances (e.g. increased redundancy)? > Specifically, shouldn't "ideal" R approximate 0.5*<sigmaI>/<I>? > > Cheers, > > Ed. > > > > -- > "I'd jump in myself, if I weren't so good at whistling." > Julian, King of Lemurs > >
