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At the very least look at unit cell packing. Usually the correct space group among possible choices is obvious from crystal packing. Phaser does a good job of placing solutions even with twinned data. A good alternative is EPMR, but we haven't needed to use it in a long time.
We had a similar low res case that was either P622 or some flavor of twinned P6 or P3. Only P312 showed reasonable crystal packing, and that solution refined to the mid 20s at 3.0 A
Roger Rowlett
Dear all,I am trying to solve the structure of a protein at ~3 A. Though the structure is already solved in a different space group. I thought it would be a straight forward MR problem. But there are a couple of problems:The possible point group is P3 (2 molecules / asu) and pointless and phenix.xtriage predicted it to be P321 ( 1 molecule /asu). But it shows 37% twinning with P3 whereas twinning is 0.055 % in P321. Several attempts to get a sensible MR solution, in either of the space groups, failed. R and Rfree were always in the range of 35-40 %. In both the cases there is a pseudotranslation of ~17%.Even different runs of the same inputs gave different solutions on multiple trials. After many trials, molrep gave one solution in P3 with 2 molecules/asu which when refined in Phaser with twinning corrections gave R and Rfree of 0.27 and 0.33 respectively. The map looks good and there is nothing really to do in terms of model-building.
While my attempts to get better dataset are ON, I was wondering how to ensure that the solution/symmetry which I got are correct (or find out that they are wrong.) and also what next with this data sets in either cases to reach to a correct model?Any suggestion would be highly appreciated.ThanksAshu