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> Alternative origins are documented in: > http://www.ccp4.ac.uk/dist/html/alternate_origins.html Note that this list makes no distinction between alternate origins and symmetry-equivalent origins. In principle, for any space group, any completely arbitrary alternate origin is permissible, all you need to do is expand the structure to P1, shift the co-ords to any arbitrary origin and off you go. It will of course mean that the symmetry elements are no longer where you expect them to be from ITC-A! In many cases the structure factor program does a partial expansion to a sub-group (maybe P1) because the SF formulae that it has coded are for a limited selection of space groups (I'm thinking specifically about SFALL, possibly other programs have a much more complete set of SF formulae coded). We try to avoid doing the full expansion because we want to take advantage of the symmetry as far as possible to minimise the computation time (though that's hardly an issue with modern computers). What this means is that only the alternate origins listed are consistent with the SF formula that the program is using for the specific space group that it uses for the calculation. However in addition, for centred space groups (A,B,C,F,I,R) some of the alternate origins will be equivalent, in that the SFs calculated using those origin shifts will be identical in both amplitude and phase so that the electron density maps will be identical, and no origin shift is needed if you want to overlay them (it may be that you still need to apply a space-group symmetry operator in order to overlay the atomic co-ordinates, but this won't change the SFs). For non-equivalent alternate origins only the calculated amplitudes are invariant, the phases are not, so that the maps will look completely different unless you apply the appropriate origin shift. For example in P212121 (0,0,0) and (1/2,1/2,1/2) are non-equivalent alternate origins (there are 8 in all) if the program uses the standard factorised SF formula for P212121, whereas in I222 these two are equivalent (so there are only 4 non-equivalent). Similar considerations apply to C222 (4 non-equivalent origins) and F222 (2) but the table above makes no such distinction. Cheers -- Ian Disclaimer This communication is confidential and may contain privileged information intended solely for the named addressee(s). It may not be used or disclosed except for the purpose for which it has been sent. If you are not the intended recipient you must not review, use, disclose, copy, distribute or take any action in reliance upon it. If you have received this communication in error, please notify Astex Therapeutics Ltd by emailing [log in to unmask] and destroy all copies of the message and any attached documents. Astex Therapeutics Ltd monitors, controls and protects all its messaging traffic in compliance with its corporate email policy. The Company accepts no liability or responsibility for any onward transmission or use of emails and attachments having left the Astex Therapeutics domain. Unless expressly stated, opinions in this message are those of the individual sender and not of Astex Therapeutics Ltd. The recipient should check this email and any attachments for the presence of computer viruses. Astex Therapeutics Ltd accepts no liability for damage caused by any virus transmitted by this email. E-mail is susceptible to data corruption, interception, unauthorized amendment, and tampering, Astex Therapeutics Ltd only send and receive e-mails on the basis that the Company is not liable for any such alteration or any consequences thereof. Astex Therapeutics Ltd., Registered in England at 436 Cambridge Science Park, Cambridge CB4 0QA under number 3751674