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RE: Providing a definition for the deprecated _symmetry_cell_setting

_symmetry_cell_setting was a most unfortunate name with an equally unfortunate definition. Getting rid of this item was one of the driving forces behind preparing the space_group dictionary. James has proposed a very elegant way of dealing with this problem. My hope is that the problem will eventually fade away. James' solution provides a way in which the presence of the deprecated item can be virtually corrected as an entry is retrieved from the archive.

David

I. David Brown
Professor Emeritus
Department of Physics and Astronomy
McMaster University
Hamilton, Ontario, Canada
From: coreDMG [coredmg-bounces@iucr.org] on behalf of James Hester [jamesrhester@gmail.com]
Sent: April 6, 2017 01:54
To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group
Subject: Providing a definition for the deprecated _symmetry_cell_setting

Dear Core DMG,

A sharp-eyed reader of the new DDLm core_CIF dictionary has noted that _space_group.crystal_system has _symmetry_cell_setting as an alias, even though _symmetry_cell_setting includes 'rhombohedral' as a possible value whereas _space_group.crystal_system does not.  As the set of possible values is different, these two data items are not strictly equivalent.  Therefore, _symmetry_cell_setting should be removed as an alias of _space_group.crystal_system.

However, we cannot simply drop _symmetry_cell_setting. For better or worse, there are over 800 CIF files in the COD which have _symmetry_cell_setting as 'rhombohedral' instead of the recommended '_space_group_crystal_system' of 'trigonal' (one has '_space_group_crystal_system' of 'rhombohedral' as well!).  Of these, over 100 were produced in the last 5 years, although the deprecation has been in place since 2005.

In order to interpret these "legacy" files, we need to preserve a definition for _symmetry_cell_setting somewhere.

I suggest that we create an additional definition in the DDLm dictionary as below.  Note that
(1) I have deliberately not provided a new dataname in <category>.<object> form, instead retaining the legacy dataname
(2) I have placed the dataname in the space_group category
(3) I have deliberately included the word deprecated in the object_id (this is only relevant for dREL methods).

Please comment.  The period for comments will be 3 weeks, after which this change will be accepted unless there are outstanding issues.

James.
======================

save_symmetry_cell_setting

_definition.id    '_symmetry_cell_setting'
_name.category_id 'space_group'
_name.object_id   'deprecated_cell_setting'
_description.text
;
     This dataname is superseded by _space_group.crystal_system and is retained here only to aid in interpreting
     legacy archives. Do not use it.

     Original definition: The cell setting for this space group symmetry.
;

_type.purpose                           State
_type.source                            Assigned
_type.container                         Single
_type.contents                          Code
loop_
  _enumeration_set.state
         triclinic          
         monoclinic         
         orthorhombic       
         tetragonal         
         trigonal 
         hexagonal          
         cubic
         rhombohedral
save_


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