Dont forget that with twinning in apparent point group PG6/mmm the true SG may be P6i or P3i21 See the twinning notes: http://www.ccp4.ac.uk/dist/html/twinning.html
Detecting twinning can be problematic - My rule of thumb, following the procedure od ctruncate:: 0) Check the matthews coefficient for likely number of molecules. Half a molecule must mean you are assigning too high a symmetry count. Lots of molecules means you need to check for non-crystallographic translation etc. 1) Look at the <I^2>/<I>^2 plot after correction for anisotropy If it isnt reasonably straight with resolution you probably have some data problems, and these can make all the tests pretty useless. 2) Is there a NC translation - truncate tells you that. If not, and the data is OK, you are unlikely to have twinning if <I^2>/<I>^2 for acentrics is ~ 2, and the L test looks OK. H test and Britten tests a bit more influenced by other NC symmetry considerations 3) If there IS NC translation <I^2>/<I>^2 for acentrics will probably be > 2 but the L test is still pretty reliable. Good luck Eleanor experimental phasing is tricky with perfect twinning but it has been done. Sorry I have forgotten reference though.. Eleanor On 29 January 2014 09:17, Kay Diederichs <kay.diederi...@uni-konstanz.de> wrote: > Dear Bert, > > as Dirk has pointed out, if P622 is the correct space group, then the > twinning statistics printed out if you process in P6 are meaningless. > > Intensity statistics, like the ratio of <I^2>/<I>^2 , can be misleading if > there is (e.g. pseudo-translational) NCS in the crystal; however, the effect > of NCS on the value of the ratio of <I^2>/<I>^2 is opposite to that of > twinning. Thus if a crystal is twinned and has NCS, you might not notice any > problem in the ratio of <I^2>/<I>^2 . > > The other statistics, like Britton and H-test, present the intensity > statistics in a different way, but from my understanding do not give > substantially different information. > > The L-test does look at a different kind of information and therefore gives > additional insight. > > If your measurements suffer from high background, diffuse scatter, ice rings, > smeared reflections, additional crystals in the beam, or any other pathology, > then all these tests may give distorted answers. In other words, even if > twinning is not really present, any test designed to convert the deviation of > data from ideality into an estimate of the twinning fraction will give you an > alpha > 0. So my experience is: if your data are very good, then the tests > give good answers; if the data are mediocre or bad, don't necessarily believe > the numbers. > > Finally, it's not only twinning of P6 that would give you P622, it's also > twinning of P3x21, P3x12 that gives P6y22. > > Hope this helps, > > Kay > > > > > On Tue, 28 Jan 2014 17:26:23 +0000, Bert Van-Den-Berg > <bert.van-den-b...@newcastle.ac.uk> wrote: > >>Dear all, >> >>I recently collected several datasets for a protein that needs experimental >>phasing. >>The crystals are hexagonal plates, and (automatic) data processing suggests >>with high confidence that the space group is P622. This is where the fun >>begins. >>For some datasets (processed in P622), the intensity distributions are >>normal, and the L-test (aimless, xtriage) and Z-scores (xtriage) suggest that >>there is no twinning (twinning fractions < 0.05). However, for other datasets >>(same cell dimensions), the intensity distributions are not normal (eg >>Z-scores > 10). Given that twinning is not possible in P622, this suggests to >>me that the real space group could be P6 with (near) perfect twinning. >> >>If I now process the "normal L-test P622" datasets in P6, the twin-law based >>tests (britton and H-test in xtriage) give high twin fractions (0.45- 0.5), >>suggesting all my data is twinned. >>Does this make sense (ie can one have twinning with "normal" intensity >>distributions)? >>If it does, would the "normal L-test" datasets have a higher probability of >>being solvable? >> >>Is there any strategy for experimental phasing of (near) perfect twins? SAD >>would be more suitable than SIR/MIR? (I also have potential heavy atom >>derivatives). >> >>Thanks for any insights! >> >>Bert >>
