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CIF core dictionary revisions List #5



Dear Colleagues,

     I enclose below the next list (#5) of changes and additions to the
core CIF dictionary for discussion and, if desired, approval.  Syd Hall
circulated comments on thirteen of the items in list #4 and I repeat these
items here together with Syd's comments and my response (and
recommendation) so that they can be discussed further.  The other items on
list #4 have been marked as approved since no one raised any objections to
them.

     Apart from the 13 items below that have been held over from list #4, all
the relatively uncontroversial changes have now been approved and it is time
to move on to the more difficult items. The first of these concerns the
adoption of items from the newly approved symmetry dictionary to replace those
in the existing symmetry category.

     Please read through this list carefully and share your comments with the
rest of the Core Dictionary Maintenance Group.

     The deadline for comment is

                    ########################
                    ### 28 February 2003 ###
                    ########################

If you have no comments, please indicate your approval (or otherwise) of the
proposed changes.  Silence will be taken as approval, though a positive
affirmative vote is much preferred.

     Please send your comments to the Core DMG list with the above subject
heading which you can most simply do by replying to this email.

			Best wishes

	                    David


*****************************************************
Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research,
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
idbrown@mcmaster.ca
*****************************************************

LIST #5 OF THE CORE REVISION PROPOSALS CONTAINS THE FOLLOWING ITEMS.  FULL
DETAILS GIVEN FOLLOWING THIS INDEX

# Items held over from list #4.
#
#     EXPTL_CRYSTAL
#        _exptl_crystal_colour   (modification)
#        _exptl_crystal_density_meas_gt
#        _exptl_crystal_density_meas_lt
#        _exptl_crystal_density_meas_temp_gt
#        _exptl_crystal_density_meas_temp_lt
#        _exptl_crystal_recrystallization
#     REFINE
#        _refine_ls_extinction_coef (replace?)
#        _refine_ls_extinction_coef_Zach
#        _refine_ls_extinction_coef_Becker-Coppens_1
#        _refine_ls_extinction_coef_Becker-Coppens_2
#        _refine_ls_extinction_BC_type
#        _refine_ls_extinction_expression (replace?)
#        _refine_ls_extinction_method (replace?)
#
# New items in this list.  These are intended to replace the current _SYMMETRY
# items and are the marked with an asterisk * below.  The full list of items
# defined in the Symmetry dictionary is given for information.
#
#     SPACE_GROUP     (General information on the space group)
#         _Space_group_Bravais_type
#         _Space_group_centring_type
#    *    _Space_group_crystal_system
#    *    _Space_group_id            (Parent to various .sg_id's)
#         _Space_group_Laue_class
#         _Space_group_IT_coordinate_system_code
#    *    _Space_group_IT_number
#    *    _Space_group_name_Hall
#         _Space_group_name_H-M_ref
#    *    _Space_group_name_H-M_alt
#         _Space_group_name_H-M_alt_description
#         _Space_group_name_H-M_full
#         _Space_group_name_Schoenflies
#         _Space_group_Patterson_name_H-M
#         _Space_group_point_group_H-M
#         _Space_group_reference_setting
#         _Space_group_transform_Pp-abc
#         _Space_group_transform_Qq_xyz
#     SPACE_GROUP_SYMOP               (Symmetry operators)
#    *    _space_group_symop_id       (parent to various .symop_id's)
#         _space_group_symop_generator_xyz
#         _space_group_symop_operation_description
#    *    _space_group_symop_operation_xyz
#    *    _space_group_symop_sg_id
#     SPACE_GROUP_WYCKOFF   (Details of the Wyckoff positions)
#         _space_group_wyckoff_coords_xyz
#         _space_group_wyckoff_id    (parent to various .wyckoff_id's to be
#                              defined)
#         _space_group_wyckoff_letter
#         _space_group_wyckoff_multiplicity
#         _space_group_wyckoff_sg_id
#         _space_group_wyckoff_site_symmetry
#
#################################################
#
#  EXPTL_CRYSTAL
#
#################################################

data_exptl_crystal_colour
    _name                      '_exptl_crystal_colour'
    _category                   exptl_crystal
    _type                       char
    _list                       both
    _list_reference            '_exptl_crystal_id'
loop_    _example
                  dark_green
                  reddish_orange_metallic
    _definition
;             The colour of the crystal used in the experiment.  The
               following list of standardized names developed for the
               International Center for Diffraction Data should be used.
               Combinations of descriptors should be linked by an underscore.

              The allowed colours are: colourless, white, black, gray, brown,
               red, pink, orange, yellow, green, blue, violet.
              Colours may be modified by using one of the prefixes: light,
               dark, whitish, blackish, grayish, brownish,reddish, pinkish,
               orangish, yellowish, greenish, bluish.
              Intermediate hues may be indicated by linking two colours,
               e.g. blue_green or bluish_green
              Metallic colours are indicated by adding 'metallic' as a suffix,
               e.g. reddish_orange_metallic for copper.
;
# COMMENT: This scheme is based on a scheme developed by the ICDD and
# recommended for use in the powder CIF.  It is already being checked
# in submissions to Acta Cryst.  It would be helpful to have it as part of the
# dictionary description, though it is difficult to define an enumeration list
# for the colours.
# STATUS: Open for discussion
#
# SUGGESTION from Syd Hall:  Split into
# _exptl_crystal_appearance_colour_primary
#         colourless, white, black, gray, brown,
#           red, pink, orange, yellow, green, blue, violet.
# _exptl_crystal_appearance_colour_modifier
#         light, dark, whitish, blackish, grayish, brownish, reddish,
#         pinkish, orangish, yellowish, greenish, bluish.
# _exptl_crystal_appearance_lustre
#    metallic, dull, clear
#
# RESPONSE: Syd's scheme would allow for enumeration lists and would avoid the
# problem of having to parse the value of this item.  Unless there are
# objections, I recommend that we adopt Syd's suggestion.

data_exptl_crystal_density_meas_*
    loop_    _name
                               '_exptl_crystal_density_meas_gt'
                               '_exptl_crystal_density_meas_lt'
    _category                   exptl_crystal
    _type                       numb
    _type_conditions            esd
    _list                       both
    _list_reference            '_exptl_crystal_id'
    _enumeration_range          0.0:
    _units                      Mg^-3^
    _units_detail              'megagrams per cubic metre'
    _definition
;              The lower (_*_gt) or upper limit (_*_lt) on the value of the
               density measured using standard chemical and physical
               methods. The units are megagrams per cubic metre (grams per
               cubic centimetre).
;
# COMMENT by Syd Hall:
# _exptl_crystal_density_meas_gt   _lt
# I don't understand what thresholds "gt" and "lt" have to do with
# density measurements. Do you simply mean the upper and lower limits
# of a range of density measurements e.g. "high" and "low", or is it
# something else?
#
# RESPONSE: Normally a measured density would be a number with an experimental
# uncertainty.  If the density were reported as lying between 2.5 and 3.5,
# _*_gt could be set to 2.5 and _*_lt to 3.5.  To give the density as 3.0(5)
# implies that 3.0 is more likely than 2.5 or 3.5, which is not what was
# reported.  In other cases the density might be reported as being greater
# than 1.0 because the sample sinks in water.  In this case _*_gt would be set
# to 1.0.  This, and the following item, were suggested by CCDC because the
# present set of items cannot be used to convey the imprecise information that
# sometimes appears in their records.  I RECOMMEND that we approve these two
# items.


data_exptl_crystal_density_meas_temp_*
    loop_
         _name                 '_exptl_crystal_density_meas_temp_gt'
                               '_exptl_crystal_density_meas_temp_lt'
    _category                   exptl_crystal
    _type                       numb
    _type_conditions            esd
    _list                       both
    _list_reference            '_exptl_crystal_id'
    _enumeration_range          0.0:
    _units                      K
    _units_detail              'Kelvins'
    _definition
;              Temperature in kelvins above which (_*_gt) or below which
               (_*_lt) _exptl_crystal_density_meas was determined.
;
# COMMENT: The above two items suggested by CCDC.
# STATUS: Open for discussion
#
# COMMENT by Syd Hall
# _exptl_crystal_density_meas_temp_gt   _lt
# I really don't know what these temperatures mean!
#
# RESPONSE: See the comment above.  The correct measurement temperature is not
# always given, but some information may be available.

data_exptl_crystal_recrystallization
    _name                      '_exptl_crystal_recrystallization'
    _category                   exptl_crystal
    _type                       char
    _example                    ?
    _definition
;              Describes the method used to recrystallize the sample
;
# COMMENT: Suggested by CCDC.
# STATUS: Open for discussion
#
# SUGGESTION by Syd Hall.  The name should be
#      _exptl_crystal_recrystallization_method
#
# RESPONSE: I RECOMMEND that we adopt Syd's suggestion.




#########################################
#
#   REFINE category
#
#########################################

# The next five items relate to extinction and are an attempt to tighten up
# our definitions.  The first item is the current definition which we may
# wish to replace or restrict.
#
# COMMENT from Syd Hall:
# _refine_ls_extinction_ etc.
#
# Please be careful with what you are advocating here... in my view it's
# moving the definitions away from the representation approach that will be
# most useful in the future. As above, enumeration is the key and what
# would be much better is to carefully enumerate _refine_ls_extinction_method
# to identify the widely used approaches. This simplifies both searching
# and validation... and future expansion for new methods.
#
# By the way, the reason for _refine_ls_extinction_expression is because
# there were a number of different "Zach" expressions in use by various
# packages at one stage... and it may well be still the case!
#
# RESPONSE: The present dictionary contains the three items
# _refine_ls_extinction_coef
# _refine_ls_extinction_expression
# _refine_ls_extinction_method
# Syd is suggesting that we provide _*_method with an enumeration list that
# covers the principal methods of correction.  If it also covers the different
# expressions used to represent them then _*_expression would no longer be
# needed except for cases that were not covered in the enumeration list.  In
# this way only a single _*_coef item would be needed and its meaning would be
# uniquely determined by the value of _*_method.
#
# The different possible values of _*_coef would all be syntactically
# equivalent, meaning that they could be read without knowing the value of
# _*_method, but _*_method would have to be consulted before a program would
# know how to use _*_coef.  This should not cause a problem with the current
# dictionary languages, but might not work with the proposed 'methods'
# dictionaries (DDL3).  Since we are still using DDL1, I recommend that we
# adopt Syd's proposal which is simpler and more elegant than the long list of
# changes I proposed below for _refine_ls_extinction_*.  If we do adopt
# this route we will need an enumeration list for _*_method containing values
# such as the following.
# Zachariasen
# Becker-Coppens_1_Gaussian
# Becker-Coppens_1_Lorentzian
# Becker-Coppens_2_Gaussian
# Becker-Coppens_2_Lorentzian
# In each case a precise expression should be defined in the dictionary.  Can
# anyone help in identifying what values should be included in this
# enumeration list and what expressions each of these values represents?
#
# I RECOMMEND that we adopt Syd's suggestion with the addition that the
# enumeration list imply the expression used (i.e. more than one enumeration
# may be needed for 'Zachariasen' if there are a number of different
# Zachariasen expressions in use).
#
###############################################################
# If Syd's recommendation is adopted, the rest of this section (the next 3
# pages) down to ####### END OF ITEMS FROM LIST #4 ####### can be ignored.  I
# have kept them in this document so that you can compare Syd's suggestion to
# the more complex suggestion that I made earlier.
###############################################################

data_refine_ls_extinction_coef
    _name                      '_refine_ls_extinction_coef'
    _category                    refine
    _type                        numb
    _type_conditions             esd
    _example                     3472(52)
    _example_detail             'Zachariasen coefficient r* = 0.347(5) E04'
    _definition
;              The extinction coefficient used to calculate the correction
               factor applied to the structure-factor data. The nature of the
               extinction coefficient is given in the definitions of
               _refine_ls_extinction_expression and
               _refine_ls_extinction_method.

               For the 'Zachariasen' method it is the r* value; for the
               'Becker-Coppens type 1 isotropic' method it is the 'g' value,
               and for 'Becker-Coppens type 2 isotropic' corrections it is
               the 'rho' value. Note that the magnitude of these values is
               usually of the order of 10000.

               Ref:  Becker, P. J. & Coppens, P. (1974). Acta Cryst. A30,
                     129-153.
                     Zachariasen, W. H. (1967). Acta Cryst. 23, 558-564.
                     Larson, A. C. (1967). Acta Cryst. 23, 664-665.
;
# COMMENT: This item is context sensitive in that it is necessary to know the
# _*_detail in order to interpret the coefficient.  Since there is no
# enumeration list for _*_detail, machine interpretation is impossible.  This
# item should be replaced for standard extinction methods by the following
# list of proposed items.  Can we retain the above definition to cover cases
# where a non-standard extinction correction was performed, or should we
# retire this name and devise a new one?  In any case we require a new
# definition.
# STATUS: open for comment

data_refine_ls_extinction_coef_*
loop_    _name                  '_refine_ls_extinction_coef_Zach'
                                '_refine_ls_extinction_coef_Becker-Coppens_1'
                                '_refine_ls_extinction_coef_Becker-Coppens_2'
    _category                    refine
    _type                        numb
    _type_conditions             esd
    _example                     3472(52)
    _example_detail              'Zachariasen coefficient r* = 0.347(5) E04'
    _definition
;        The extinction coefficient used to calculate the correction
              factor applied to the observed structure-factors.

         For _refine_ls_extinction_coef_Zach (the Zachariasen, also known as
              the Larson, method) the value given is the r* value,

         For _refine_ls_extinction_coef_Becker-Coppens_1 (the Becker-Coppens
              type 1 isotropic method) it is the 'g' value,

         For _refine_ls_extinction_coef_Becker-Coppens_2 (Becker-Coppens type
              2 isotropic method) it is the 'rho' value.

         Note that the magnitude of these values is usually of the order of
              10000.

         The Becker-Coppens procedure is referred to as 'type 1' when
             correcting secondary extinction dominated by the mosaic spread;
             as 'type 2' when secondary extinction is dominated by particle
             size and includes a primary extinction component; It is also
             possible to use a mixture of types 1 and 2.

         For the Becker-Coppens method it is necessary to set the
             mosaic distribution as either 'Gaussian' or 'Lorentzian' in
             _refine_ls_extinction_B-C_type.  If an anisotropic extinction
             correction has been made the multiple coefficients cannot be
             contained in *_extinction_coef_Becker-Coppens_* and they must be
             listed in _refine_special_details.

               Ref:  Becker, P. J. & Coppens, P. (1974). Acta Cryst. A30,
                     129-153.
                     Zachariasen, W. H. (1967). Acta Cryst. 23, 558-564.
                     Larson, A. C. (1967). Acta Cryst. 23, 664-665.
;
# COMMENT: This is a new item which allows the specific extinction parameter
# to be given for each of the standard extinction corrections.  Whichever item
# is given will identify the method used to determine the extinction so
# _*_expression and _*_method would not be needed.
# STATUS: Open for discussion.

data_refine_ls_extinction_BC_type
    _name               '_refine_ls_extinction_BC_type
    _category            refine_ls
    _type                        char
loop_    _related_item
          '_refine_ls_extinction_coef_Becker-Coppens_1'
          '_refine_ls_extinction_coef_Becker-Coppens_2'
    _related_function          ?
loop_    _enumeration       Gaussian
                            Lorentzian
    _definition
;           For the Becker-Coppens extinction correction, a statement is
            needed of the function used to describe the mosaic spread.

               Ref:  Becker, P. J. & Coppens, P. (1974). Acta Cryst. A30,
                     129-153.
;
# COMMENT: This is a new item is needed to complement the Becker-Coppens
# coefficients
# STATUS: Open for discussion


data_refine_ls_extinction_expression
    _name                      '_refine_ls_extinction_expression'
    _category                    refine
    _type                        char
#    _example
#;                         Larson, A. C. (1970). "Crystallographic Computing",
#                             edited by F. R. Ahmed. Eq. (22) p. 292.
#                             Copenhagen: Munksgaard.
#;
# A new example will be needed if the revised definition is adopted.
    _definition
;            A description or reference for the extinction correction equation
             used to apply the data item _refine_ls_extinction_coef when this
             is not one of the standard types defined under
             _refine_ls_extinction_coef_*.
             This information must be sufficient to reproduce the extinction
             correction factors applied to the structure factors.
;
# COMMENT: This is an existing data item but its name suggests an algebraic
# expression while the example suggests that a reference is intended.  I have
# changed the definition so maybe a different dataname is needed.  It is not
# clear to me what the difference between _*_expression and _*_method was
# intended to be.
# STATUS: open for discussion.

data_refine_ls_extinction_method
    _name                      '_refine_ls_extinction_method'
    _category                    refine
    _type                        char
#    _enumeration_default        'Zachariasen'
#    loop_ _example              'B-C type 2 Gaussian isotropic'
#                                'none'
    _definition
;            A description of the extinction correction method applied with
             the data item _refine_ls_extinction_coef when one of the standard
             methods is not used.
# (The rest of this text should be deleted or transferred. The text above has
# been modified.  The enumeration default should in any case be deleted. and
# a different example given.)
#
#           This description should
#             include information about the correction method, either
#             'Becker-Coppens' or 'Zachariasen'. The latter is sometimes
#             referred to as the 'Larson' method even though it employs
#             Zachariasen's formula.
#
#             The Becker-Coppens procedure is referred to as 'type 1' when
#             correcting secondary extinction dominated by the mosaic spread;
#             as 'type 2' when secondary extinction is dominated by particle
#             size and includes a primary extinction component; and as 'mixed'
#             when there is a mixture of types 1 and 2.
#
#             For the Becker-Coppens method it is also necessary to set the
#             mosaic distribution as either 'Gaussian' or 'Lorentzian'; and
#             the nature of the extinction as 'isotropic' or 'anisotropic'.
#             Note that if either the 'mixed' or 'anisotropic' corrections
#             are applied the multiple coefficients cannot be contained in
#             *_extinction_coef and must be listed in _refine_special_details.
#
#               Ref:  Becker, P. J. & Coppens, P. (1974). Acta Cryst. A30,
#                     129-153.
#                     Zachariasen, W. H. (1967). Acta Cryst. 23, 558-564.
#                     Larson, A. C. (1967). Acta Cryst. 23, 664-665.
;
#
# COMMENT: The default should be deleted since there should be no presumption
# about the method used.  This proposal changes the meaning of this item in a
# significant way and perhaps a different dataname should be used.
# STATUS: Open for comment
#
######## END OF ITEMS FROM LIST #4 #################################

######## NEW ITEMS IN LIST #5 ######################################
#
# NEW PROPOSAL
#
# Recently COMCIFS approved the symmetry dictionary which was designed to
# provide a more complete and coherent list of symmetry items than is
# currently available in the core dictionary.  While the current _symmetry_*
# items have proved satisfactory for reporting crystal structures, they are
# not suitable for further expansion of symmetry properties and in some cases,
# such as _symmetry_cell_setting, the names are quite misleading.
# The proposal is to replace the core items:
# _symmetry_cell_setting
# _symmetry_equiv_pos_as_xyz
# _symmetry_Int_Tables_number
# _symmetry_space_group_name_Hall
# _symmetry_space_group_name_H-M
# by some or all of the items in the list below.
#
#  This list gives all the items and categories defined in the
# symmetry dictionary.  Those marked with * are proposed for inclusion in the
# new version of the core dictionary.  The definitions of the asterisked items
# are given in full in the list below.
# Definitions of the remaining items can be found in the full
# symmetry dictionary (written in DDL2) available on the IUCr web site.
#
#
#     SPACE_GROUP     (General information on the space group)
#         _Space_group_Bravais_type
#         _Space_group_centring_type
#      *  _Space_group_crystal_system
#      *  _Space_group_id            (Parent to various .sg_id's)
#         _Space_group_Laue_class
#         _Space_group_IT_coordinate_system_code
#      *  _Space_group_IT_number
#      *  _Space_group_name_Hall
#         _Space_group_name_H-M_ref
#      *  _Space_group_name_H-M_alt
#         _Space_group_name_H-M_alt_description
#         _Space_group_name_H-M_full
#         _Space_group_name_Schoenflies
#         _Space_group_Patterson_name_H-M
#         _Space_group_point_group_H-M
#         _Space_group_reference_setting
#         _Space_group_transform_Pp-abc
#         _Space_group_transform_Qq_xyz
#     SPACE_GROUP_SYMOP               (Symmetry operators)
#      *  _space_group_symop_id       (parent to various .symop_id's)
#         _space_group_symop_generator_xyz
#         _space_group_symop_operation_description
#      *  _space_group_symop_operation_xyz
#      *  _space_group_symop_sg_id
#     SPACE_GROUP_WYCKOFF   (Details of the Wyckoff positions)
#         _space_group_wyckoff_coords_xyz
#         _space_group_wyckoff_id    (parent to various .wyckoff_id's to be
#                           defined)
#         _space_group_wyckoff_letter
#         _space_group_wyckoff_multiplicity
#         _space_group_wyckoff_sg_id
#         _space_group_wyckoff_site_symmetry
#
################################################
#
#           CATEGORY: SPACE_GROUP
#
################################################

data_SPACE_GROUP
    _name                  space_group
    _category              category_overview
    _type                  nul
    _description
;              Contains all the data items that refer to the space group as a
               whole, such as its name, Laue group etc.  It may be looped, for
               example, in a list of space groups and their properties.

               Space group types are identified by their International Tables
               for Crystallography Vol A number or Schoenflies symbol.
               Specific settings of the space groups can be identified by
               their Hall symbol, by specifying their symmetry operations or
               generators, or by giving the transformation that relates the
               specific setting to the reference setting based on
               International Tables for Crystallography Vol. A and stored in
               this dictionary.

               The commonly-used Hermann-Mauguin symbol determines the
               space group type uniquely but several different Hermann-Mauguin
               symbols may refer to the same space group type. It contains
               information on the choice of the basis, but not on the
               choice of origin.
;
    _example
;
           _space_group_id                 1
           _space_group_name_H-M           'C 2/c'
           _space_group_name_Schoenflies   C2h.6
           _space_group_IT_number          15
           _space_group_name_Hall          '-C 2yc'
           _space_group_Bravais_type       mS
           _space_group_Laue_class         2/m
           _space_group_crystal_system     monoclinic
           _space_group_centring_type      C
           _space_group_Patterson_name_H-M 'C 2/m'
;


data_space_group_crystal_system
    _name              '_space_group_crystal_system'
    _category          space_group
    _list              both
    _list_mandatory    no
    _list_reference    '_space_group_id'
    _description
;              The name of the system of geometric crystal classes of space
               groups (crystal system) to which the space group belongs.
               Note that crystals with the hR lattice type belong to the
               trigonal system.
;
    _type              char
     loop_
    _enumeration
                                  triclinic
                                  monoclinic
                                  orthorhombic
                                  tetragonal
                                  trigonal
                                  hexagonal
                                  cubic
    _related_item        '_symmetry_cell_setting'
    _related_function    replace

data_space_group_id
    _name                '_space_group_id'
    _category            space_group
    _list                yes
    _list_mandatory      yes
    _description
;              This is an identifier needed if _space_group_* items are looped.
;
    _type                char
     loop_
    _list_link_child
                         '_space_group_symop_sg_id'
                         '_space_group_Wyckoff_sg_id'

data_space_group_IT_number
    _name                 '_space_group_IT_number'
    _category             space_group
    _list                 both
    _list_mandatory       no
    _list_reference       '_space_group_id'
    _description
;              The number as assigned in International Tables for
               Crystallography Vol A, specifying the proper affine class (i.e.
               the orientation preserving affine class) of space groups
               (crystallographic space group type) to which the space group
               belongs.  This number defines the space group type but not
               the coordinate system in which it is expressed.
;
    _type                 numb
    _enumeration_range    1:230
    _related_item         '_symmetry_Int_Tables_number'
    _related_function     replace

data_space_group_name_Hall
    _name                 '_space_group_name_Hall'
    _category             space_group
    _list                 both
    _list_mandatory       no
    _list_reference       '_space_group_id'
     loop_
    _example
    _example_detail       'P 2c -2ac'            'Equivalent to Pca21'
                          '-I 4bd 2ab 3'         'Equivalent to Ia3d'
    _description
;              Space group symbol defined by Hall (Acta Cryst. (1981) A37,
               517-525) (See also International Tables for Crystallography
               Vol.B (1993) 1.4 Appendix B).
               Each component of the space group name is separated by a
               space or an underscore.  The use of space is strongly
               recommended.  The underscore is only retained because it
               was used in earlier archived files.  It should not be
               used in new CIFs.
               _space_group_name_Hall uniquely defines the space group and
               its reference to a particular coordinate system.
;
    _type                 char
    _related_item          '_symmetry_space_group_name_Hall'
    _related_function     replace

data_space_group_name_H-M_alt
    _name                  '_space_group_name_H-M_alt'
    _category               space_group
    _list                   both
    _list_mandatory         no
    _list_reference       '_space_group_id'
    _type                   char
     loop_
    _example
    _example_detail
;
     loop_
    _space_group_name_H-M_alt
    _space_group_name_H-M_alt_description
     'C m c m(b n n)' 'Extended Hermann-Mauguin symbol'
     'C 2/c 2/m 21/m' 'Full unconventional Hermann-Mauguin symbol'
     'A m a m'        'Hermann-Mauguin symbol corresponding to setting used'
;
'three examples for the space group number 63.'

    _description
;              *_name_H-M_alt allows for an alternative Hermann-Mauguin symbol
               to be given.  The way in which this item is used is determined
               by the user and should be described in the item
               _space_group_name_H-M_alt_description.  It may, for example, be
               used to give one of the extended Hermann-Mauguin symbols given
               in Table 4.3.1 of International Tables for Crystallography
               Vol A (1995) or a full Hermann-Mauguin symbol for an
               unconventional setting.
               Each component of the space group name is separated by a
               space or underscore.  The use of space is strongly
               recommended.  The underscore is only retained because it
               was used in earlier archived files.  It should not be
               used in new CIFs.  Subscripts should appear
               without special symbols. Bars should be given as negative
               signs before the numbers to which they apply. The commonly
               used Hermann-Mauguin symbol determines the space group type
               uniquely but a given space group type may be described by
               more than one Hermann-Mauguin symbol. The space group type
               is best described using the *_IT_number or *_name_Schoenflies.
               The Hermann-Mauguin symbol may contain information on the
               choice of basis though not on the choice of origin. To
               define the setting uniquely use *_name_Hall, list the
               symmetry operations or generators, or give the transformation
               that relates the setting to the reference setting defined
               in this dictionary under *_reference_setting.
;

    _related_item      '_symmetry_space_group_name_H-M'
    _related_function  replacement

###########################################################
#
#     SPACE GROUP SYMOP
#
###########################################################

data_SPACE_GROUP_SYMOP
    _category                  category_overview
    _type                      nul
    _description
;              Contains information about the symmetry operations of the
               space group.
;
     loop_
    _example_detail
    _example
;
    The symmetry operations for the space group P21/c
;
;    loop_
    _space_group_symop_id
    _space_group_symop_operation_xyz
    _space_group_symop_operation_description
  1    x,y,z          'identity mapping'
  2   -x,-y,-z        'inversion'
  3   -x,1/2+y,1/2-z  '2-fold screw rotation with axis in (0,y,1/4)'
  4    x,1/2-y,1/2+z  'c glide reflection through the plane (x,1/4,y)'
;

data_space_group_symop_id
    _description
;              An arbitrary identifier that uniquely labels each symmetry
               operation in the list.
;
    _type              char
    _name              '_space_group_symop_id'
    _category          space_group_symop
    _list              yes
    _list_mandatory    yes
    _related_item      '_symmetry_equiv_pos_site_id'
    _related_function   replace



data_space_group_symop_operation_xyz
    _name              '_space_group_symop_operation_xyz'
    _category          space_group_symop
    _list              both
    _list_mandatory    no
    _list_reference    '_space_group_symop_id'
     loop_
    _example
    _examples_detail
              'x,1/2-y,1/2+z'  'c glide reflection through the plane (x,1/4,z)'
    _description
;               A parsable string giving one of the symmetry operations of the
                space group in algebraic form.  If W is a matrix representation
                of the rotational part of the symmetry operation defined by the
                positions and signs of x, y and z, and w is a column of
                translations defined by the fractions, an equivalent position
                X' is generated from a given position X by the equation:

                          X' = WX + w

                (Note: X is used to represent bold_italics_x in International
                Tables for Crystallography Vol. A, Section 5)

                When a list of symmetry operations is given, it is assumed
                that the list contains all the operations of the space
                group (including the identity operation) as given by the
                representatives of the general position in International
                Tables for Crystallography Vol. A.
;
    _type              char
    loop_
    _related_item
    _related_function
  '_symmetry_equiv_pos_as_xyz'          replace
  '_space_group_symop_generator_xyz'    alternate
    _item_default_value   'x,y,z'

data_space_group_symop_sg_id
    _name              '_space_group_symop_sg_id'
    _category          space_group_symop
    _list              both
    _list_mandatory    no
    _list_reference    _space_group_symop_id
     loop_
    _example
    _example_detail
                          ?      ?
    _description
;               A child of _space_group.id allowing the symmetry operation
                to be identified with a particular space group.
;
    _type              numb
    _list_link_parent  '_space_group.id'

# COMMENT: See above.  The _space_group items above have all been approved by
# COMCIFS.  The only question we have to decide is which ones are to be included
# in the revised dictionary.
#
# STATUS: Open for discussion


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