Not quite. I was trying to say that for good small molecule data, R1 is usally significantly less than Rmerge, but never less than the precision of the experimental data measured by 0.5*<sigmaI>/<I> = 0.5*Rsigma (or the very similar 0.5*Rpim).
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 Thu, 28 Oct 2010, Jacob Keller wrote: > 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" > <gshe...@shelx.uni-ac.gwdg.de> > To: <CCP4BB@JISCMAIL.AC.UK> > 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: j-kell...@northwestern.edu > ******************************************* > >
