Ooops bad link: http://img.chem.ucl.ac.uk/sgp/medium/monoc.htm
-- Ian On 2 July 2012 11:49, Ian Tickle <ianj...@gmail.com> wrote: > Hi James > > I would say that you are completely correct in your assessment of the > situation. The Bilbao server clearly lists only the unique 'standard' > settings, leaving to the user the task of rotating the basis to > whatever is required by the unit cell convention, or other criteria > such as isomorphism with historical structures. > > The decision to list only the standard settings was adopted by > editions of ITC up to 1974; the 1983 and all later editions list some > additional possible conventional settings, but still omit the settings > such as the one in your example where the unit cell contains > additional centred lattice points. > > The rule adopted prior to the 1983 edition to omit all the > non-standard settings and to leave it to the reader to generate the > desired conventional setting from the corresponding standard setting > has led to the unfortunate misconception among many crystallographers > that the standard settings are the conventional ones, whereas in fact > the IUCr rule is (and always has been) that the conventional setting > is primarily determined by the point group and the unit cell lengths > and only secondarily by the symmetry elements of the space group. > > The UCL server differs from the 1983 ITC in that it lists all possible > conventional settings (see for example for monoclinic: > http://img.chem.ucl.ac.uk/sgp/medium) including the one with > additional lattice points such as B121. BTW the B2 designation is > ambiguous: it could mean B211 or B112 (both # 5) as well as B121 # 3 > (also I assume you meant to say that B121 is # 3, not # 2, which is > P-1 ?). > > The story goes that the reason for the original 'standard settings > only' rule adopted by the 1952 and earlier editions of ITC was to save > paper: during WW2 and the years immediately following there was a > paper shortage in the UK where ITC was published (Kynoch Press, > Birmingham), and most of it had to be imported from Canada. With the > advent of the Internet I would say that saving paper is no longer a > relevant consideration and there's no longer any excuse not to list > (and indeed make use of) all settings! > > Cheers > > -- Ian > > On 29 June 2012 21:10, James Stroud <xtald...@gmail.com> wrote: >> Hello Everyone and especially symmetry experts, >> >> I found at http://img.chem.ucl.ac.uk/sgp/medium/003dy1.htm that B2 (SG #2) >> is a recognized space group that has a basis rotated from P2 (and as a >> result has two new positions). >> >> I'm wondering whether a similar B2 for P2 (LG #3) is recognized for the >> layer groups? I checked wikipedia (http://en.wikipedia.org/wiki/Layer_group) >> and the Bilbao server (http://www.cryst.ehu.es/subperiodic/get_sub_gen.html) >> and could find no evidence of a B2 layer group being recognized, but I can >> also come up with no logical reason why it shouldn't be recognized. >> >> Thank you for any input, >> >> James >> >> -- >> James Stroud >> >> http://www.jamesstroud.com >>
