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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: Absolute structure****From**:**syd@crystal.uwa.edu.au (Sydney R Hall)***Date*:*Wed, 1 Apr 1998 15:10:08 +0100 (BST)**Reply-To*:*coredmg@iucr.org*

I like the sound of this data item and support it. It will certainly be a useful addition to the Acta C required items. I have fiddled with the human readable definition a bit... partly cosmetic but also to make it clearer what this measure is in terms of existing data items. data_reflns_Friedel_coverage _name '_reflns_Friedel_coverage' _category reflns _type numb _enumeration_range 0.0:1.0 _definition ; The proportion of Friedel related reflections present in the number of the "independent reflections" specified by the item _reflns_number_total This proportion is calculated as the ratio: [N(Crystal class) - N(Laue symmetry)] / N(Laue symmetry) where, working from the _diffrn_refln_, list N(Crystal class) is the number of reflections obtained on averaging under the symmetry of the crystal class N(Laue symmetry) is the number of reflections obtained on averaging under the Laue symmetry. Examples: (a) For centrosymmetric structures its value is necessarily equal to 0.0 as the crystal class is identical to the Laue symmetry. (b) For whole-sphere data for a crystal in the space group P1, _reflns_Friedel_coverage is equal to 1.0, as no reflection h k l is equivalent to -h -k -l in the crystal class and all Friedel pairs {h k l; -h -k -l} have been measured. (c) For whole-sphere data in space group Pmm2, the value will be < 1.0 because although reflections h k l and -h -k -l are not equivalent when h k l indices are non-zero, they are when l=0. (d) For a crystal in the group Pmm2 measurements of the two inequivalent octants h >= 0, k >=0, l lead to the same value as in (c), whereas measurements of the two equivalent octants h >= 0, k, l >= 0 will lead to a zero value for _reflns_Friedel_coverage. ; Cheers, Syd. Howard, I wish I could talk you out of using your mime mailer. Apart from generating many lines of rubbish at this end, the fragmentation of the "definition" section really made reading your stuff difficult and created quite a bit of work putting it back into a coherent block of text. There are other modern mailers that don't incur this sort of overhead! :-(

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