The original reference for the H cell is the very first edition of Int. Tab.:
Hermann, C. (1935). Internationale Tabellen zur Bestimmung von Kristallstrukturen. Berlin: Gebrueder Borntraeger. Cheers -- Ian On Wed, 22 Jul 2020 at 17:34, Eleanor Dodson < 0000176a9d5ebad7-dmarc-requ...@jiscmail.ac.uk> wrote: > 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 > ######################################################################## 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/
