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Re: Absolute structure



After more thought and consideration of George's comments on counting
reflections and the completeness calculation for non-centrosymmetric
structures, I think that the most useful way forward is 
 - to drop the proposed _reflns_number_Friedel
 - to go back to the 2.0.1 definition of _reflns_number_total, apart
from expanding the explanation of the concept of a 'Friedel equivalent'
reflection
 - include _gt and _observed to correspond to _total
 - to add a new item which might be called _reflns_Friedel_completeness,
which can take values between 1.0 (bad) and 2.0 (good), and which is
defined as being the quotient of the number of independent reflections
after averaging under the crystal class divided by the number of
independent reflections after averaging under the Laue symmetry. For
centrosymmetric structures this parameter is by definition equal to 1
and there is no need to report it. The question in the _reflns_number_ 
and the _measured_fraction_ items of the exact interpretation of
"unique" and "symmetry-independent" is left wide open. Whether these
refer to the crystal class or the Laue symmetry is a question of taste.
Indeed in the _measured_fraction items one might even take one choice
for the numerator and one for the denominator. 
 As I see things now, twenty years ago the vast of majority of data sets
had a Friedel_completeness of 1.0. In twenty years time the vast
majority of data sets will have a Friedel_completeness of 2.0. At the
moment we are stuck in between. Soon Friedel's law for
non-centrosymmetric structures will be regarded as the exception rather
than the rule.

Here is the complete set of items which I am now proposing:

data_reflns_Friedel_completeness
    _name                        '_reflns_Friedel_completeness'
    _category                    reflns
    _type                        numb
    _enumeration_range           1.0:2.0
    _definition
;              Working from the _diffrn_refln_ list,
               _reflns_Friedel_completeness is defined as being the
quotient
               of the number of reflections after averaging under the
crystal
               class divided by the number of reflections after
               averaging under the Laue symmetry. 
;


data_reflns_number_total
    _name                        '_reflns_number_total'
    _category                    reflns
    _type                        numb
    _enumeration_range           0:
    _definition
;              The total number of reflections in the _refln_ list (not
the
                _diffrn_refln_ list). It may include Friedel equivalent
               reflections (i.e. those which are symmetry equivalent
under the
               Laue symmetry but inequivalent under the crystal class) 
               according to the nature of the structure and the
procedures 
               used. The item _reflns_special_details  describes the
reflection
               data.
;



data_reflns_number_gt
    _name           '_reflns_number_gt'
    _category       reflns
    _type           numb
    _enumeration_range      0:
    _definition     
;              The number of reflections in the _refln_ list (not the
               _diffrn_refln_ list) that are significantly intense,
               satisfying the criterion specified by
               _reflns_threshold_expression. It may include Friedel
               equivalent reflections (i.e. those which are symmetry
equivalent
               under the Laue symmetry but inequivalent under the
crystal class)
               according to the nature of the structure and the
procedures used.
               The item _reflns_special_details describes the reflection
data.
;


data_reflns_number_observed
    _name                      '_reflns_number_observed'
    _category                    reflns
    _type                        numb
    _related_item              '_reflns_number_gt'
    _related_purpose             replace
    _enumeration_range           0:
    _definition
;              The number of 'observed' reflections, in the _refln_ list
(not
               the _diffrn_refln_ list). The observed reflections
satisfy the
               threshold criterion specified by
_reflns_threshold_expression
               (or the deprecated _reflns_observed_criterion). It may
               include Friedel equivalent reflections according to the
               nature of the structure and the procedures used. The item
               _reflns_special_details describes the reflection data.
;


data_reflns_number_Friedel
   REMOVE THIS PROPOSED ITEM

-- 
Howard Flack        http://www.unige.ch/crystal/ahdf/Howard.Flack.html
Laboratoire de Cristallographie               Phone:(+41 22) 702 62 49
24 quai Ernest-Ansermet             mailto:Howard.Flack@cryst.unige.ch
CH-1211 Geneva 4, Switzerland                   Fax:(+41 22) 781 21 92


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