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*To*:*Multiple recipients of list <coredmg@iucr.org>***Subject**:**Re: Absolute structure****From**:**Howard Flack <Howard.Flack@cryst.unige.ch>***Date*:*Tue, 10 Feb 1998 17:12:33 GMT**Reply-To*:*coredmg@iucr.org*

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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