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Re: Modus operandi of COMCIFS Core Dictionary Maintenance Group



	Yuri is right to emphasise the need for crystal clear definitions. 
However, recommendations need to be used with care as I was reminded by
Howard earlier.  Cif is a permissive standard that allows a user to do
anything that is legal.  It is not prescriptive in saying that there is
only one correct way to present information.  Where cif recommends one
item over another, it is stating IUCr policy that has been recommended by
one of the Union's Commissions.  For example, the Commission on
Nomenclature recently recommended that atomic displacement parameters
should be given as U rather than B.  However, the macromolecular community
current uses B extensively.  B is not illegal and is provided for in cif,
but with the recommendation of the Commission that U is preferred.

	Particular users of cif may make their own rules about what fields
should be used.  Some users may not be interested in atomic displacement
parameters and will ignore B and U when given.  Others may require that
the cif contains only U or only B.  Acta Cryst. C now effectively requires
authors submitting cifs to use U.  Thus, a paper that presents this
information in the form of B can be returned to the author for correction. 
This is Acta Cryst. C policy but is not a policy required by cif.  Another
journal may require B to be presented in spite of the Commission
recommendation.  All of these are within the prerogative of the user -
Comcifs places no restrictions.  Zeit. Krist., for example, can specify
exactly which cif items it requires for an acceptable paper.  The journal
does not have to make allowances for all the possible variations allowed
by cif. 

				David

*****************************************************
Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
idbrown@mcmaster.ca
*****************************************************



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