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Re: proposal for valence items



Howard,

	There is a way of getting your defintion of chirality into CIF
along the following lines:

data__chemical_chirality
 _name				_chemical_chirality
 _category			chemical
 _type				char
 _definition
;  A flag indicating that the crystal contains a chiral molecule.
   Chirality is defined as the geometric property of a rigid object (or
spatial arrangement of points or atoms) of being nonsuperposable on its
mirror image; such an object has no symmetry operations of the second kind
(a mirror plane, m; a centre of inversion, -1; a roto-inversion axis, -N).
If the object is superposable on its mirror image, the object is described
as being achiral. 
;
loop
 _enumeration
 -enumeration_detail
			yes  'a chiral molecule is present'
			no   'no chiral molecule is present'

This item could appear in a CIF though there may never be any need to
introduce it, but in any case the definition can be referred to from other
items in the dictionary (or elsewhere) and so perform the function you are
looking for.

	I don't know what other items you want to define, so I can't
comment on whether the same trick would work for them.  Is it worth
thinking again about adding these definitions?

				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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