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Re: Absolute structure

  • 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)
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. 

    _name                      '_reflns_Friedel_coverage'
    _category                    reflns
    _type                        numb
    _enumeration_range           0.0:1.0
;              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.

                 (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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