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Enumeration ranges

  • To: COMCIFS@iucr
  • Subject: Enumeration ranges
  • From: Brian McMahon <bm@iucr.org>
  • Date: Mon Nov 24 15:27:23 1997
H> Within the current CIF dictionary definitions, it is not possible to
H> express the above interval of x correctly in _enumeration range. I have
H> taken the liberty of breaking the rules in order to express my intent.
H> 
H> data_refine_ls_abs_structure_Flack
H>     _name                      '_refine_ls_abs_structure_Flack'
H>     _category                    refine
H>     _type                        numb
H>     _type_conditions             esd
H>     _enumeration_range           -3.0*u:1.0+3.0*u
H>     _definition
H> ;            The measure of absolute structure (enantiomorph or polarity) as
H>              defined by Flack.
H>  
H>              For centrosymmetric structures the only permitted value, if the 
H>              data name is present, is 'inapplicable' represented by '.' .
H> 
H>              For non-centrosymmetric structures the value must lie in the
H>              99.97% Gaussian confidence interval  -3u =< x =< 1 + 3u and a
H>              standard uncertainty (e.s.d.) u must be supplied.
H> 
H>              Ref: Flack, H. D. (1983). Acta Cryst. A39, 876-881.
H> ;

I've changed the wording of the definition. The enumeration range I have
left unchanged as 0.0:1.0, because - as Howard points out - the formalism is
not sufficiently well developed to provide an unambiguous machine-parsable
instruction to validation software. It may be that this can be done through
association of "methods" (i.e. procedures expressed in some agreed computer
language or metalanguage); there is already provision for such an approach
in DDL2, but it is as yet only a placeholder - I'm not aware of any well
developed applications that use embedded methods

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