Seems to me the sensible approach is the systematic H-M symbol defined in syminfo.lib, i.e. the first letter is always the centring symbol (PIRFABCH) and then if there's any ambiguity the basis can be indicated with a : appendage, so R3:H or R3:R etc, then H3 et al is reserved for H-centred P3 et al. In reality there is never any ambiguity between R3:H and R3:R, as you said it's always obvious from the cell which is the appropriate basis. I've learned from bitter experience never to trust the SG symbol anyway! - if it's R3, R32, H3, H32 I always test the cell and reset the symbol if necessary! If anyone starts using H centring that will break my code since I can't distinguish between H3 (PDB) and H3 (IUCr): they can't of course since H centring is (thankfully) never used in syminfo.lib. But as you point out there is still the potential for confusion in publications.
Historically I believe that R3 was first defined as the hexagonal basis with R centring, which would be logical. Then later on some crystallographers decided they preferred the primitive rhombohedral cell with the 3-fold along the body diagonal; logically this should have been called P3, except that the name was obviously already taken, so they continued to call it R3 even though it was P not R centred. This was perhaps not such a good idea in hindsight. So historically the vast majority of crystallographers used R3 to mean R3:H, and only a small minority ever actually used it to mean R3:R (for a long time most software never supported the latter option anyway, maybe a lot of it still doesn't). So it's unfortunate that the PDB's change from R3 to H3 inconveniences by far the largest group of users! - so I think remedying the historical mistake of using R3 to mean primitive rhombohedral cell by making it R3:R (or maybe P3:R would be more logical), and keeping with R3 for the R centred, hexagonal obverse basis would have been wiser! The other big inconvenience that was thrust on us was of course the possibility that SG names could contain spaces. The pain that this caused was totally unnecessary because the original set of SG symbols without embedded spaces were already unique, even if you threw the additonal set of long symbols (e.g. P1211) in with the short ones (P21). Even if the issue was merely one of legibility (P 1 21 1 is slightly easier to read), this could have been handled by mapping internally stored versions without spaces to the user-visible versions when printing log files etc. -- Ian > -----Original Message----- > From: Bernhard Rupp [mailto:[EMAIL PROTECTED] > Sent: 28 October 2008 05:22 > To: Ian Tickle; [email protected] > Subject: RE: [ccp4bb] H32 use > > Good point. Btw, also these nonstandard, > triple ab-plane centered H3x cells can be > readily reindexed standard primitive, so no new > space groups are needed...let's not give the PDB > new ideas... > > BR > > -----Original Message----- > From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On > Behalf Of Ian > Tickle > Sent: Monday, October 27, 2008 7:59 PM > To: [email protected] > Subject: Re: [ccp4bb] H32 use > > In fact there's another good reason not to use H3 etc when > you mean R3: > the H lattice symbol is already in use to mean something completely > different! - namely H-centring in *trigonal & hexagonal* (i.e. *not > rhombohedral*) space groups such as P3, P3/1 etc & > supergroups, so e.g. > H3 is actually a supergroup of P3 not R3. > > See here: http://books.google.co.uk/books?id=ilVvOYOFCx8C&pg=PA96 > > and here: http://img.chem.ucl.ac.uk/sgp/large/trigonal.htm > > and the above is consistent with the definition now adopted in the new > ITC-A1: > > http://it.iucr.org/cgi-bin/itsearch?query=DC.creator%3D%22H.%2 > 2%20AND%20 > DC.creator%3D%22Wondratschek%22&metaname=swishdefault&IT.group > =2.1.4&IUC > r.volume=A1 > > It would be interesting to know if the PDB consulted the IUCr on this > change! > > -- Ian > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:[EMAIL PROTECTED] On Behalf Of Bernhard Rupp > > Sent: 27 October 2008 22:50 > > To: [email protected] > > Subject: H32 use > > > > Dear All, > > > > I wonder whether it is a good idea to still accept H32 as a space > > group in the PDB CRYST1 record, despite it may be used > internally by > > programs. > > > > * The combination of hexagonal cell parameters and R32 clearly > > indicates a hexagonal (obverse) axis setting in the R centered cell. > > > > * The H-M R32 symbol with rhombohedral setting a, alpha, is > > inconsistent > > anyhow because the cell is then primitive and not centered. > > > > * The ITCA (and most data mining programs?) seem to be unaware of a > > Bravais symbol H. > > > > The practice of using H32 anywhere in published data should be > > strongly discouraged. Internally please feel free to use whatever > > works. > > > > What is the current opinion on this? > > > > Best, BR > > ----------------------------------------------------------------- > > Bernhard Rupp > > 001 (925) 209-7429 > > +43 (676) 571-0536 > > [EMAIL PROTECTED] > > [EMAIL PROTECTED] > > http://www.ruppweb.org/ > > ----------------------------------------------------------------- > > The hard part about playing chicken > > is to know when to flinch > > ----------------------------------------------------------------- > > > > > > > Disclaimer > This communication is confidential and may contain privileged > information > intended solely for the named addressee(s). It may not be > used or disclosed > except for the purpose for which it has been sent. If you are not the > intended recipient you must not review, use, disclose, copy, > distribute or > take any action in reliance upon it. 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