Well - yes . I am a true devotee of doctored cells to match something already in existence in a higher symmetry which has become approximate in some new manifestation! But I hadnt realised there were official versions of doctoring.. Eleanor
On Wed, 22 Jul 2020 at 16:29, Jeremy Karl Cockcroft <jeremyk...@gmail.com> wrote: > Dear Eleanor, > What you say is absolutely spot-on! An H3 cell can be reduced down to the > smaller P3 cell as you pointed out. > > However, sometimes it may be useful to use a larger unit cell. I can't > give an example for trigonal space groups or from protein crystallography, > but in a recent paper, I used a unit cell in C-1 (i.e. a doubled P-1 > triclinic cell) as this related to the C-centred monoclinic cell as > exhibited in a higher temperature phase. I could have used P-1, but I knew > that chemists would see the relationship between the phases more easily by > using an enlarged cell. I have done this sort of thing many times, e.g. I > used F2/d for the low temperature phase of DI (HI) many years ago instead > of C2/c as this related to the face-centred cubic form. As I am interested > in phase transitions, I tabulated a range of space-group settings for > enlarged unit cells on my site. > > I am not sure that this will make the CCP4 list as I am not subscribed to > it - please feel free to echo it on there. > Best regards, > Jeremy Karl. > *************************************************************** > Dr Jeremy Karl Cockcroft > Department of Chemistry > (University College London) > Christopher Ingold Laboratories > 20 Gordon Street > London WC1H 0AJ > +44 (0) 20 7679 1004 (laboratory) > +44 (0) 7981 875 829 (cell/mobile) > j.k.cockcr...@ucl.ac.uk or jeremyk...@gmail.com > http://img.chem.ucl.ac.uk/www/cockcroft/homepage.htm > *************************************************************** > 6 Wellington Road > Horsham > West Sussex > RH12 1DD > +44 (0) 1403 256946 (home) > *************************************************************** > > > On Wed, 22 Jul 2020 at 14:51, Nicholas Keep <n.k...@mail.cryst.bbk.ac.uk> > wrote: > >> >> >> >> -------- Forwarded Message -------- >> Subject: Re: [ccp4bb] Question about P3, H3 and R3 space groups >> Date: Wed, 22 Jul 2020 14:41:27 +0100 >> From: Eleanor Dodson <0000176a9d5ebad7-dmarc-requ...@jiscmail.ac.uk> >> <0000176a9d5ebad7-dmarc-requ...@jiscmail.ac.uk> >> Reply-To: Eleanor Dodson <eleanor.dod...@york.ac.uk> >> <eleanor.dod...@york.ac.uk> >> To: CCP4BB@JISCMAIL.AC.UK >> >> 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/
