Dear Dhaval, In principle, it does not matter how you describe the packing of the molecules in your crystal. E.g. you can refine a P212121 structure in space group P212121 with one molecule in the asymmetric unit, or in P1 with 4 molecules in the asymmetric unit. In the latter case, these 4 molecules are related by non-crystallografic symmetry operators, which are identical to the crystallographic symmetry operators in P212121 (provided of course, that the same origin was choosen in both cases.).
So if your molecule is a tetramer with an internal 4-fold axis, the crystal packing could be described in P4212 with one molecule in the asymmetric unit, or in P212121 with 2 molecules in the asymmetric unit. In the latter description, the 4-fold axis is non-crystallographic. (I did not check though, how this works out with 2-fold versus 21 axes). In general, if there is no reason to assume lower symmetry, e.g. too high Rfactors during refinement, one generally chooses the highest symmetry, since there is no reason to assume that the molecules are different. However, if there are clear differences during refinement in the different space groups, one chooses the space group which yields the lowest Rfactors. Beware that in lower symmetry, Rfactors might be somewhat lower, just because the refinement program has more parameters to fiddle with. This is, in my eyes, not significant. Best, Herman Von: CCP4 bulletin board [mailto:[email protected]] Im Auftrag von dhaval patel Gesendet: Freitag, 22. Mai 2015 06:21 An: [email protected] Betreff: Re: [ccp4bb] Point group Even I have the same problem my crystals get processed in to P212121 as well as P4212 and I am currently analysing the data but not able to conclude the space group. Any idea? Both structure while refining is giving around same R/Rfree. Any help will be appreciable. Dhaval Patel PhD Student, Bioinformatics & Structural Biology Indian Institute of Advanced Research +91-9925450504 On Thu, May 21, 2015 at 10:57 PM, frazao <[email protected]<mailto:[email protected]>> wrote: On 05/21/2015 05:23 PM, Keller, Jacob wrote: I can process my 3.8 A dataset in either P4 or P422 point groups. Do the scaling statistics look similar for both? If so, go with P422. Trying to enforce an incorrect symmetry operator would blow up your stats. ...But in the case of twinning, lower space groups can masquerade as higher ones. I say try out P4 with twin refinement, see how your stats go. JPK Or eventually even to further lower symmetry... I had once an example of true P21212 crystals that processed nicely as P4212, due to a twinning two-fold axis parallel to the a,b diagonal Carlos -- ************************************** Dr. Carlos Frazao Structural Biology Laboratory - Macromolecular Crystallography Unit ITQB-UNL, Av Republica, Apartado 127 2781-901 Oeiras, Portugal Phone: (351)-214469666/609 FAX: (351)-214433644 e-mail: [email protected]<mailto:[email protected]> www.itqb.unl.pt<http://www.itqb.unl.pt>
