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Just to clarify this again: the question of "standard" settings for primitive orthorhombic space groups (& the I2/C2 choice) was raised by Ian Tickle in earlier discussions. He pointed out that the IUCr standard was to choose a < b < c for primitive orthorhombic irrespective of which axes are 2 or 2(1), and to choose either I2 or C2 to give the smaller beta angle his references http://nvl.nist.gov/pub/nistpubs/jres/106/6/j66mig.pdf http://nvl.nist.gov/pub/nistpubs/jres/107/4/j74mig.pdf This makes sense as you can generally make a < b < c at the indexing stage (unless some lengths are equal) before you know which axes are 2 or 2(1), and then you don't have to reindex later. For the I2/C2 choice, it is somewhat more convenient to have a less oblique axis system if you can. I made POINTLESS follow these conventions by default, but you do have a choice. These settings should then be referred to as the "standard" settings (ie a<b<c, CELL-BASED in Pointless), and the alternative (SYMMETRY-BASED in Pointless) may be called "reference" settings (eg always P21 21 2 for space group 18). Most crystallographic software that we use is happy with non-reference space groups eg space group P2 21 2 : all CCP4 and Phenix programs are, as far as I know. The exceptions which don't work (and that I know about) are:- 1) Arp/warp: Victor Lamzin has I believe promised (Victor?) that the imminent new released version will work for these settings 2) Sharp: Clemens Vonrhein told me that he hoped the next release of Sharp would handle these cases I know this is a bit inconvenient at present, but I thought it a Good Thing to follow recognised conventions (and to force software developers to make their programs more general :-)) Phil Evans On 18 Aug 2009, at 12:48, Michele Lunelli wrote: > Dear Arefeh, > > Arp/warp works with standard space groups, therefore you have to > reindex P21221 to P21212. In this > message from Anastassis Perrakis you can find some advice: > https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind0709&L=CCP4BB&T=0&F=&S=&P=100954 > > > Michele > > > > Arefeh Seyedarabi wrote: >> Hi, >> >> I have a question; recently I have encountered a few space groups >> which are >> not supported by Refmac5. Examples include I2 and P21221. Both >> these space >> groups have been identified as the best solutions for the two >> different >> datasets I am working on using Pointless. However, I am faced with >> difficulties in Refmac5, and the program fails to complete when I >> select >> refinement cycles with Arp-waters...with the message saying 'space >> group not >> supported'. >> >> Any suggestions on how this problem could be overcome? >> >> Regards, >> >> Arefeh >>