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So I guess a consequence of what you say is that since in cases where there is no solvent the R values are often better than the precision of the actual measurements (never true with macromolecular crystals involving solvent), perhaps our real problem might be modelling solvent? Alternatively/additionally, I wonder whether there also might be more variability molecule-to-molecule in proteins, which we may not model well either. JPK ----- Original Message ----- From: "George M. Sheldrick" <[log in to unmask]> To: <[log in to unmask]> Sent: Thursday, October 28, 2010 4:05 AM Subject: Re: [ccp4bb] Against Method (R) > 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 >> >> ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program Dallos Laboratory F. Searle 1-240 2240 Campus Drive Evanston IL 60208 lab: 847.491.2438 cel: 773.608.9185 email: [log in to unmask] *******************************************