But surely P3 symmetry ' cell ( a,a,c 90 90 120) with origin shifts (⅓,⅔,0), +(⅔,⅓,0) can just be indexed as P3 with cell (a/sqrt(3), a/sqrt(3), c, 90 90 120) ?? Eleanor
On Wed, 22 Jul 2020 at 14:32, Nicholas Keep <n.k...@mail.cryst.bbk.ac.uk> wrote: > I have an answer from Jeremy Cockcroft the author of the 'Birkbeck' > Tables. Actually Jeremy has been at UCL for a decade or so and they are > hosted from their > > Nick > > In answer to the question regarding the use of R and H for trigonal space > groups, the letters refer to two distinct types of lattice centring > symmetry operation, namely +(⅓,⅔,⅔), +(⅔,⅓,⅓) and +(⅓,⅔,0), +(⅔,⅓,0), > respectively. For the subset of rhombohedral space groups, the symbol R > should always be used. When the alternative unit cell with a=b=c and > α=β=γ is chosen, this cell corresponds to a primitive Bravais lattice, but > the label P is not used as this would result in confusion with > non-rhombohedral space groups. (The choice of symmetry operators for > rhombohedral space groups is wholly dependent of the choice of unit cell, > i.e. hexagonal versus rhombohedral, so there is no ambiguity if the unit > cell is specified.) > > For the non-rhombohedral space groups, it may occasionally be convenient > to choose a larger H-centred unit cell that is not the usual primitive P > one. (The use of larger unit cells is quite common for systems that > undergo phase transformations as it may enable the crystallographer to keep > the contents of the unit cell the same in both phases.) Note that the > use of an H-centred lattice switches the order of the symmetry elements in > these space group symbols, e.g. P312 becomes H321. > > I am not aware of any changes to this convention, which I believe has a > long history. However, it is possible that the letter H has been used > unwittingly for other purposes. > > Jeremy Karl Cockcroft > > > -- > > NOTE NEW PHONE NUMBER JULY 2020 > > Prof Nicholas H. Keep > Executive Dean of School of Science > Professor of Biomolecular Science > Crystallography, Institute for Structural and Molecular Biology, > Department of Biological Sciences > Birkbeck, University of London, > Malet Street, > Bloomsbury > LONDON > WC1E 7HX > > Office G54a > > Dean Email; scid...@bbk.ac.uk > Dept email n.k...@mail.cryst.bbk.ac.uk > Telephone 020-3926-3475 (Will contact me at home if working as well as my > office) > > If you want to access me in person you have to come to the crystallography > entrance > and ring me or the department office from the internal phone by the door > > > ------------------------------ > > To unsubscribe from the CCP4BB list, click the following link: > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
