You can always re-index a monoclinic unit cell with beta greater than 120deg.
Usually the beta closer to 90deg is preferred, but either will work.
Bernie Santarsiero
On May 21, 2014, at 11:54 AM, Roberto Battistutta wrote:
> Dear All,
> a question about C2 and I2 space groups.
> Processing a dataset, XDS output says C2, with dimensions 122.8, 56,9 81,5 and beta 125.1°.
> Aimless reindexes to I2 with 81.6, 56.9, 101.1 and 96.2°.
> Phenix (refine) returns a warning "NOTE: non-standard setting used: I 1 2 1".
> In the PDB there are indeed several examples of I2 choices.
>
> The two space groups are equivalent as indicated in the International Tables (all with number 5), but why different programs use different conventions? Is there any rationale? By the way, the volumes of the two unit cells are very similar.
>
> Thanks,
> Roberto.
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