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: data_refine_diff_density_ loop_ _name '_refine_diff_density_max' '_refine_diff_density_min' '_refine_diff_density_rms' _category refine _type numb _type_conditions esd _units e_A^-3^ _units_detail 'electrons per cubic angstrom' _definition ; 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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