Maximum theoretical drop R/Rfree for perfect twin from 30% is around 25% (i.e. it could go down to 5%). However it could only happen only if twinning is perfect and there is no pseudo rotation parallel to twin operator. Hypothetical case it can happen if you have refined one crystal structure at sufficiently high resolution till (almost convergence) and another crystal is twinned but otherwise perfectly isomorphous to the first crystal and you take coordinates from the first crystal and refine against the second crystal.
regards Garib On 10 Feb 2011, at 20:14, Patskovsky Yury wrote: > Dear all, > > > Twin refinement has yielded Rwork/Rfree values of about 0.10/0.12 for > a nice quality 1.8A dataset (Rmerge 6%, space group I4, twin fractions > 0.6/04) and almost the same R/Rfree (0.095/0.115) for another 1.5A nice > quality data set (Rmerge 6%, space group I4, twin fractions 0.74/0.26). > Refinement of untwinned data resulted in Rfree of ~32% and ~22% respectively. > REFMAC and PHENIX both have produced the same results and almost identical R > factors, which are suspiciously VERY LOW for this resolution of data. Twin > refinement in REFMAC has produced exceptional quality maps even for 1.8A data > (they look rather like 1.2A maps) - I can not tell the same for PHENIX - > maps were looking worse (may be someone has a better idea why). > Normally twin refinement results in lowering R-factors - say, the drop > in R from 30% (without twin refinement) to 20% (with twin refinement) would > be considered normal, however we can see the drop from 32% to 12%. > I wonder if anyone else has experienced similar problems and what > would be the most reasonable explanation for that. > > > Thank you > > Yury > > > > > > Yury Patskovsky, Ph.D. > Associate, > Dept of Biochemistry > Albert Einstein College of Medicine > 1300 Morris Park Ave > Bronx, NY 10461 > phone 718-430-2745 > yu...@medusa.vioc.aecom.yu.edu
