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magCIF - policy advice requested

  • To: "Discussion list of the IUCr Committee for the Maintenance of the CIF Standard (COMCIFS)" <comcifs@iucr.org>
  • Subject: magCIF - policy advice requested
  • From: James Hester <jamesrhester@gmail.com>
  • Date: Tue, 27 May 2014 17:20:50 +1000
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Dear COMCIFS members and advisers:

I am pleased to advise that a CIF dictionary for description of
magnetic structures (magCIF) is currently in preparation and it is
expected that a final draft could be ready before the IUCr meeting.
This has raised a policy issue for COMCIFS that we need to deal with
in a timely way.

By its nature, the magCIF dictionary builds on the definitions in the
core CIF dictionary, modulated structures CIF dictionary, and symmetry
CIF dictionary (including extending looped categories).  At the same
time, the authors wish it to be a single, coherent document.  Core CIF
and the modulated structures dictionary use DDL1 naming conventions,
whereas symCIF is a DDL2 dictionary with DDL2 naming conventions. For
coherence and convenience, the authors of magCIF should clearly use a
single DDL and naming convention.

My inclination is to recommend writing magCIF using DDL2.
Semantically, this will mean that certain DDL2 concepts (e.g. 'key')
will be implicitly imposed on DDL1 datanames.  This mapping is however
straightforward and implied by the presence of 'aliases' in mmCIF and
other DDL2 dictionaries

More trivially, this approach will result in some loops that have
names not containing a period mixed with names that do contain a
period, and non-looped datanames in the CIF data file will also
contain mixtures of such names. I note that the use of a period to
separate category and item is purely conventional and is not
syntactically or semantically required by the DDL that the dictionary
is written in, so I do not consider this to be a problem.

A further advantage of DDL2-style names is that when magCIF is
translated into DDLm at some not-too-distant point, the same names can
be used (as DDLm naming conventions are the same as DDL2 naming
conventions) and software written with the DDL2 magCIF dictionary in
mind will not require updating to handle files written against the
'new' DDLm magCIF.

Does anybody see any issues with this recommendation?

James.


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