There is a good description of this situation in the paper:
"Conventional Cells—The Last Step Toward General Acceptance of Standard Conventional Cells for the Reporting of Crystallographic Data"
published in J. Res. Natl. Inst. Stand. Technol. 107, 373–377 (2002).
The conclusion and recommendation quoted from this paper is "...In all cases then, the practice for the monoclinic system would be to select a conventional cell based on the shortest vectors in the ac plane (b-axis unique) for primitive as well as centered cells."
Hope this helps.
Antonio
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Dr. Antonio L. Llamas-Saiz
Unidade de Raios X. RIAIDT
Edificio CACTUS. Campus Vida
Universidade de Santiago de Compostela
E-15782 Santiago de Compostela
SPAIN
Tel.: +34 881 816 223
Fax.: +34 881 816 203
E-mail: [log in to unmask]
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-----Original Message-----
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Phil Evans
Sent: Wednesday, May 21, 2014 9:07 PM
To: [log in to unmask]
Subject: Re: [ccp4bb] C2/I2 space groups
I2 and C2 are different settings of the same group. The official IUCr convention is to use the one which gives the beta angle closer to 90 degrees. As far as I know all programs should now be able to use the I2 setting, but if it worries you, you can reindex to C2
Phil
On 21 May 2014, at 17:54, Roberto Battistutta <[log in to unmask]> 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.
>
>
> Roberto Battistutta
> Associate Professor
> Department of Chemistry
> University of Padua
> via Marzolo 1, 35131 Padova - ITALY
> tel. +39.049.827.5262
> fax. +39.049.827.5829
> [log in to unmask]
> www.chimica.unipd.it/roberto.battistutta/
> VIMM (Venetian Institute of Molecular Medicine) via Orus 2, 35129
> Padova - ITALY tel. +39.049.7923.236 fax +39.049.7923.250 www.vimm.it
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