Re: [ccp4bb] Layer Groups: B2 recognized?
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
Re: [ccp4bb] Layer Groups: B2 recognized?
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
[ccp4bb] Layer Groups: B2 recognized?
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
