Using SHELXL, restraints can be applied to symmetry equivalent atoms so one can easily restrain the sulphate to be a regular tetrahedron by restraining the S-O distances to be equal and the O1..O2, O1'..O2 etc. distances to be equal (with or without target values). For details of a similar example see Acta Cryst. A64 (2008) 112-122. From Eleanor's comment it appears that REFMAC can now do this too, though I was not aware of the facility. If you then wish to refine S anisotropically SHELXL will also automatically generate and apply the constraints required for the special position (in any space group), this can be very useful for high-resolution refinements.
George Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-2582 On Fri, 11 Jan 2008, Charlie Bond wrote: > Eleanor Dodson wrote: > > It should be more or less equivalent, but better I think to put 1S at > > 0.5 occ and 2O at occ = 1 > > > > At least in REFMAC the restraints to the symmetry atoms should be set up > > But if you do that, some of the bond angles will be unrestrained, won't > they?: O1-S-O2 will be OK, but O1-S-O1' and O2-S-O2' will not. Or is > refmac cleverer than I think? > > Cheers, > Charlie > -- > Charlie Bond > Professorial Fellow > University of Western Australia > School of Biomedical, Biomolecular and Chemical Sciences > M310 > 35 Stirling Highway > Crawley WA 6009 > Australia > [EMAIL PROTECTED] > +61 8 6488 4406 >
