I can see no advantage in insisting on c-unique in the primitive orthorhombic space groups. The indexing program will generally (always?) choose a<b<c, so then when you determine the space group as say P 2 21 21, P 21 2 21, or P 21 21 2 there is no need to reindex. If there is an indexing ambiguity, as with a ~= b, or always in space groups such as P3x, you will always _have_ to resolve the ambiguity (except for the first crystal), either using reference (e.g. in XDS or Pointless), or for serial crystallography using a clever algorithm such as the one Kay et al. developed
Also this is the recommended Best Practice:
http://nvlpubs.nist.gov/nistpubs/jres/107/4/j74mig.pdf
(doi: http://dx.doi.org/10.6028/jres.107.030)
http://nvlpubs.nist.gov/nistpubs/jres/106/6/j66mig.pdf
(doi: http://dx.doi.org/10.6028/jres.106.050)
As far as I know, all programs we use are happy with e.g. space group P 2 21 21, apart from XDS which only refers to space groups by number, e.g. an ambiguous 18 for P 21 21 2 and its permutations.
Phil
> On 29 Jan 2016, at 09:13, Graeme Winter <[log in to unmask]> wrote:
>
> Good morning all,
>
> It is with some trepidation I raise the following question: does anyone still care about reindexing orthorhombic lattices so that the unique axis is C? I.e. P21221 => P21212
>
> Back in the day certain programs would express unhappiness if you fed them P21 2 21 (say) data - I am certain that this problem has gone away. Is there any reason in 2016 that (say) xia2 should write out symmetry based not cell based data? I am leaning towards indexing these so that a < b < c and then the screw axes are whatever they are.
>
> How do people feel about this?
>
> Thanks & best wishes Graeme
>
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