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Re: Neutron diffraction experiments

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: Neutron diffraction experiments
  • From: syd@crystal.uwa.edu.au (Sydney R Hall)
  • Date: Wed, 18 Feb 1998 17:15:19 GMT
Howard's latest comments highlight the hardest part about the data 
definition business and I make this observation after my tenth year 
in it. There is never a shortage of opinions in an electronic forum 
such as this... especially retrospective ones about existing definitions! 
And as often or not they are in the direction of increased pedantry. 
Perhaps that is not such a bad thing (though one has to cringe at some 
of the recent discussions on absolute structure) but I do have a genuine 
concern that comcifs is heading in the same direction as the sorts of
discussions that led to the replacement of 'esd' with 'su'. As Howard
was involved in this esoteric exercise, such concerns may have some 
foundation. These are indeed added to by the references to the semantic
wisdom of the chair of the nomenclature commission, that body being
involved in this exercise. This leads me to remind recent comcifs 
appointees of two important aspects of past activities. 

(1) The original cif core definitions represent the combined wisdom 
of a number of learned crystallographers a decade ago. They are 
intended to be as simple as possible so that they can be understood 
by the non-expert. Certainly they can be improved upon but it seems
less than synergistic to be making wholesale changes to their wording
or to dwell too long on the particular semantics. I really doubt that
it is useful for each generation of comcifs members to redefine data 
items which are well understood and widely used within the community...
as these particular ones are! In the light of past criticism about 
the length of time it takes to get Permitting new physical units approved and 
released it also seems to be counter-productive role.

(2) In late 1996 there was a considerable debate about the involvement
of the nomenclature commission in comcifs activities, and the possibility
of having cif definitions "approved" by that body. The majority view
within comcifs and the then iucr executive was that this would not be 
appropriate, and that the functions of the two bodies are quite different. 
There was an important message in that ruling about the purpose of data
definitions and the need for new items to keep pace with the rapidly 
changing demands of the community.

Which brings us back to the data items in question. It would appear that 
the present definition, sans the references to electron density and units,
will suffice for probes other than x-rays. This remains my preferred option.

Who will have first crack at rewording the existing definition:

    loop_ _name                '_refine_diff_density_max'
    _category                    refine
    _type                        numb
    _type_conditions             esd
    _units                       e_A^-3^
    _units_detail              'electrons per cubic angstrom'
;              The largest, smallest and root-mean-square-deviation, in
               electrons per angstrom cubed, of the electron density in the
               final difference Fourier map. The *_rms value is measured with
               respect to the arithmetic mean density, and is derived from
               summations over each grid point in the asymmetric unit of
               the cell. This quantity is useful for assessing the
               significance of *_min and *_max values, and also for
               defining suitable contour levels.

Cheers, Syd.

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