To add a little more clarity to the discussion. The hexagonal obverse setting is 3X larger because it is triply primitive. This means there are 3 lattice points per unit cell, similar to the face or body centered lattices, which have two lattice points per cell.
To read BR's scripts: For the primitive Rhombohedral setting: a=b=c means a is equal to b is equal to c. al=be=ga <> 90° means alpha is equal to beta is equal to gamma, but not necessarily equal to 90°. As Harry points out, all angles can be equal to 90°, but they don't have to be. For the triply primitive Hexagonal setting: a=b, means that a is equal to b and c is unique. al=be=90°, ga=120° means that alpha and beta are equal to 90°, and gamma is 120°. Notice that the triply primitive hexagonal setting, as its name implies, has the same unit cell requirements as a hexagonal lattice. I was taught when writing the space group symbol for a manuscript the lattice type is italic, the symmetry operators are normal, and the screw axes are subscript. All are written without spaces between them. As BR points out, entry into programs is at the discretion of the program author. Of course, the International Tables has a wonderful discussion on all this and I encourage you to read it to fully understand the nomenclature. HTH, Chris On 7/27/12 3:21 PM, "Harry" <[email protected]> wrote: >Hi > >Just to clarify - for H32, a=b<>c (though you could have a=b=c within >measurement error), alpha=beta=90º, gamma=120º. Just having alpha=beta ><> gamma is necessary but not sufficient. > >BTW, there's nothing wrong with alpha=beta=gamma=90º for the R >setting. It's just not required. > >Sorry if this isn't clear - I'm just watching a very odd Olympic >opening ceremony, and it's quite distracting! > >On 27 Jul 2012, at 21:09, Bernhard Rupp wrote: > >> H32 indicates the hexagonal obverse setting (as you list) for a R >> centered trigonal cell, which is 3x larger than the primitive R32 >> cell indexed a=b=c, al=be=ga <> 90. Standard imho is the H32 >> setting, for which I will probably get flamed. >> The relation between H and R cells is depicted here: >> >>http://www.ruppweb.org/Garland/gallery/Ch5/pages/Biomolecular_Crystallogr >>aphy_Fig_5-29.htm >> >> This has been discussed and is explained in the ccp4 tutorials and >> doc afaik, where you can find more detailed info. >> >> For proper format in a journal, I would suggest to adhere to the >> format given in the ITC (International tables for Crystallography), >> I.e. Bravais Italic, subscripted screw symbols. Note that this is >> not the format you put it into most programs - their docs help. >> >> You can also try my old space croup decoding program to see general >> positions, operators, matrices and other useful stuff. >> >> http://www.ruppweb.org/new_comp/spacegroup_decoder.htm >> >> HTH, BR >> >> -----Original Message----- >> From: CCP4 bulletin board [mailto:[email protected]] On Behalf >> Of Theresa Hsu >> Sent: Friday, July 27, 2012 12:54 PM >> To: [email protected] >> Subject: [ccp4bb] Space group R32 and H32 >> >> Dear all >> >> I have a confusion on the space group R32 and H32. For a cell >> parameter of a = b not equal to c, alpha=beta, not equal to gamma, >> is it considered as R32 or H32? >> >> I tried searching the mail list archives but it does not help a >> beginner crystallographer like me. >> >> I also have another basic question. What is the correct way for >> writing space groups? Is the Bravais lattice in italic and is there >> space after that? Or it does not matter because both are used in >> literature? >> >> Thank you. > >Harry >-- >Dr Harry Powell, MRC Laboratory of Molecular Biology, MRC Centre, >Hills Road, Cambridge, CB2 0QH >
