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Hi all One should be aware of some stats about twinning (and in general): 1) If you have perfect hemihedral twinning and no correlated rotational NCS then random R factors are around 40 instead of Luzzati's 58. 2) If you have perfecthemihedral twinning and you are not modelling twin then random R factor is around 50 (again instead of 58). It may confuse things after molecular replacement. 3) In twinning case reflections are related with each other. refmac groups them together so that Rfree is unbiased by twinning. However after non-twin refinement if you start twin refinement then Rfree in the first few cycles can be small. If you use consistenlty refmac with twin refinement then Rfree should be fine Intuitive reason for the first two phenomena is that if the crystal is twinned then the distrbution of the data becomes narrower and properties like Rfactor that are dependent on the spread of the distribution become smaller. (There is a mathematical formulation that gives the exact numbers also). Presence of pseudo translation (broadens the distribution) and rotation (data points are not independent) complicates things a bit further. The reason for the third phenomenon should e clear. Unfortunately statistics like Rfactor are not robust to violations of such assumptions. You can get very interesting Rfactors under different assumptions. For me the best indicator is the electron density (do you see new features and can you remove wrong bits) and chemical sensibility of the electron density you see. Garib On 14 Aug 2008, at 07:46, Jan Abendroth wrote: > Hi all, > kind of a weird problem - the R-factors of a refinement using the > new twin refinement in refmac are low, almost suspiciously low: > A good 1.9AA data set, space group H3/R3, many statistics starting > with truncate's cumulative intensity distribution clearly suggest > twinning. The structure (SSGCID target) was solved by MR, very > rigid beta-helix (see 3bxy, very cool fold!), maps nice. I use > refmac 5.5.0046 for refinement, with and without the new TWIN flag. > > Here my concerns: > Despite 0.3/0.7 twin domains as suggested by various programs and > refined by refmac, the difference of between the Rs of the twinned > refinement and the non-twinned refinement are constantly rather > little: 0.143/0.174 for the twinned case, 0.218/0.275 for the non- > twinned case. This seems rather low to me for a quite high twin > fraction. Plus the R-factors for the twinned case are really low > for a 1.9AA structure. As expected, the maps from the twin > treatment are a bit nicer that for the non-twinned treatment. > Any reasons for concerns or just a very rigid structure that > refines really well or refmac handling twinning really nicely? > > Thanks for any input > Jan > > -- > Jan Abendroth > deCODE biostructures > Seattle / Bainbridge Island WA, USA > work: JAbendroth_at_decode.is > home: Jan.Abendroth_at_gmail.com >