Hi Eleanor H3 #143 is (and always has been) a triply-primitive cell setting of P3.
http://img.chem.ucl.ac.uk/sgp/large/143bz1.htm Cheers -- Ian On Wed, 22 Jul 2020 at 14:42, Eleanor Dodson < 0000176a9d5ebad7-dmarc-requ...@jiscmail.ac.uk> wrote: > 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 > ######################################################################## 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/
