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Re: CIF atoms

This is not a niaive question, but an important question with subtle and
interesting implications.  The short answer is:  no, the atom sites in a
loop do not have to belong to the same connected molecule, but the loop
has to contain sufficient information so that it is possible to determine
the molecule with which each atom is to be associated.  This problem
arises frequently in mmCIF data sets, and is resolved by having chain
identifiers.  One might think of having, say, multiple loops, one per
molecule, but in order to do that, you would have to start another full
data block, since all the rows of loops for a given category within a
given data block belong within the same loop.  One assumes you want to
bring multiple molecules together in order to show some
inter-relationship.  In that case a common loop with tags to help separate
the fragments makes a lot of sense.

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On Tue, 24 Jul 2001, Owen Johnson wrote:

> I will probably regret asking such a niaive question, but do all
> atom sites in a loop have to belong to the same connected molecule?
> I can see obvious advantages if they do, but can someone point to
> a specification or ruling? Thanks
> --
> Owen Johnson  Systems Group, Cambridge Crystallographic Data Centre
> 12 Union Road, CAMBRIDGE CB2 1EZ UK  Web: http://www.ccdc.cam.ac.uk
> Tel:(44) 01223 336023  Fax:(44) 01223 336033  Enq:(44) 01223 336408