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Providing a definition for the deprecated _symmetry_cell_setting

  • To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group <coredmg@iucr.org>
  • Subject: Providing a definition for the deprecated _symmetry_cell_setting
  • From: James Hester <jamesrhester@gmail.com>
  • Date: Thu, 6 Apr 2017 15:54:22 +1000
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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.



_definition.id    '_symmetry_cell_setting'
_name.category_id 'space_group'
_name.object_id   'deprecated_cell_setting'
     This dataname is superseded by _space_group.crystal_system and is retained here only to aid in interpreting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

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