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 --------------------------------------------------------------------- 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: [email protected] --------------------------------------------------------------------- -----Original Message----- From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of Phil Evans Sent: Wednesday, May 21, 2014 9:07 PM To: [email protected] 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 <[email protected]> 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 > [email protected] > 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
