Dear Kai, you are right, but the original question was posed in the context of xia2. At that stage I think only diffraction data are available to decide about screw axes. Best, Herman
-----Ursprüngliche Nachricht----- Von: Kay Diederichs [mailto:kay.diederi...@uni-konstanz.de] Gesendet: Freitag, 29. Januar 2016 16:16 An: Schreuder, Herman R&D/DE; CCP4BB@JISCMAIL.AC.UK Betreff: Re: AW: [ccp4bb] Spacegroups, screw axes and ordering Dear Herman, I did not write anything about the question of assigning a spacegroup based on screw axes absences. _Of course_ this is unreliable, but this was not the question (to me, at least). And of course I agree that one should try molecular replacement with all possible space groups, unless the assignment is clear. The question, as I understand it, was whether _if you know where the screw axis is and where not_ if you then want a) to order the axes such that a < b < c , and call the space group e.g. P 21 2 21 (because in that axis ordering, it is the b axis that does not have a screw), or, b) if you want to have the "unique" axis last, giving P 2 2 21 or P 21 21 2 , respectively, and accept the fact that this may result in a > b or b > c . And my preference is choice b) for reasons I tried to explain. best, Kay On 2016-01-29 15:09, herman.schreu...@sanofi.com wrote: > Dear Kai, > > I agree that Ian's response was strongly worded, but I don't think it is > personal. I agree with Ian that assignment of screw axes based on axial > reflections is very unreliable. That is why I always advice people to run > molecular replacement in all possible space groups, independent of what the > data processing programs may claim. If a data set gets reindexed because > molecular replacement found out that the initial assignment of screw axes was > incorrect, this is also a great source of confusion, especially if novices > follow the advice of the bulletin board to always use the original mtz file > as input for refinement! > > Best, > Herman > > -----Ursprüngliche Nachricht----- > Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Kay > Diederichs > Gesendet: Freitag, 29. Januar 2016 13:51 > An: CCP4BB@JISCMAIL.AC.UK > Betreff: Re: [ccp4bb] Spacegroups, screw axes and ordering > > Ian, > > we have not met in the past and did not have any scientific conflict, AFAIK. > > Thus, I am really surprised that you attack my opinion so verbosely, > with strong words such as "seriously misrepresenting" and "asserting". > Maybe a cultural or language difference of some sort that I have not so > far encountered, nor do I understand it. > > Anyway, I won't try to answer in the same or similar way. > > sincerely, > Kay > -- Kay Diederichs http://strucbio.biologie.uni-konstanz.de email: kay.diederi...@uni-konstanz.de Tel +49 7531 88 4049 Fax 3183 Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz This e-mail is digitally signed. If your e-mail client does not have the necessary capabilities, just ignore the attached signature "smime.p7s".
