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Re: Approval of CIF core changes for 2.3



BMcM> I haven't deleted the Z_prime entry, though you have indicated
BMcM> that you will pull it from consideration at this stage (though I
remark that
BMcM> there have been requests from other sources for a record of Z', so
finding a
BMcM> sound definition would be a useful priority for the next version).

  I sort of agree.

  The above Z' is rather crude. What is really required, according to
me, is a CIF encoding for the molecular symmetry (crystallographic and
non-crystallographic) and chirality of all molecules in the asymmetric
unit. It seems that this must be in a loop so that one knows which
symmetry belongs to which type of molecule. It is too much to ask that
this be implemented in the current revision.

  On the other hand many structures only have one type of molecule in
them. In this case the suggested Z' would serve a purpose if the wording
could be beefed up to make it clear that it only deals with the
one-type-of-chemical-molecule case. Very many structures are like this
and so the CIF item would be useful. I'm sure that the editors of Acta
Cryst C and E could fairly easily be convinced that such information
should become obligatory immediatetly in their journals. For the moment
the more complicated cases could be dealt with by text in the body of
the article or a CIF comment item.

  My counter example is that of a 'co-crystal' in which there are
several different independent molecules. There was a poorly attended
meeting at the IUCr Congress in Geneva which started to discuss
nomenclature for co-crystals. The chair of the nomenclature commission
was there. The prime mover is located at the CCDC. I wonder what is
going on.

BMcM> Nor have
BMcM> I included either of Howard's alternative definitions for
BMcM> _space_group_symop_operation_xyz

  I'm not surprised. The problem is to know which one is correct. I did
not look but perhaps there is some small print in the symmetry cif dic
which makes it clear. I do not have time myself to look. 

H.

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