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Re: Approval of CIF core changes for 2.3



Part 2 of comments from HDF. 


>#################################################
>#
>#  EXPTL_CRYSTAL
>#
>#################################################


HDF           US             GB
HDF         color           colour
HDF         gray            grey
HDF   You have a foul mixture.



>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'
>    _related_item              '_exptl_crystal_density'
>    _related_function          alternate
>    _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
HDF;              The value above which (_*_gt) or below which (_*_lt)
the
HDF               density measured using standard chemical and physical
HDF               methods lies. The units are megagrams per cubic metre
(grams per
>               cubic centimetre).  These items should not be used to report
>               new experimental work where _exptl_crystal_density_meas should
>               be used.  They are intended for use in reporting information in
>               existing databases and archives which would be misleading if
>               reported under _exptl_crystal_density_meas.

> '_exptl_crystal_density_meas_temp_lt   300'
>  ;          The density was measured as some unspecified temperature below room 
HDF;          The density was measured at some unspecified temperature
below room 
>           temperature.


># COMMENT: Submitted by Brian McMahon  This identifier will allow linking to
># other publications or files by the contact author. 
># (Linking based on name alone is inefficient because
># individuals change their names or spelling of the names.  A complementary
># data name is proposed for the authors in the PUBL_AUTHOR category.

HDF In addition it would be nice to have a complimentary data name for
authors.


>data_SPACE_GROUP
>    _name                  space_group
>    _category              category_overview
>    _type                  null
>    _description
>;              Contains all the data items that refer to the space group as a
>               whole, such as its name, crystal system etc.  It may be looped,
>               for example, in a list of space groups and their properties.
>
>               Only a subset of the space_group category items appear in the 
>               core dictionary.  The remainder are found in the cif_sym
>               dictionary.
>
>               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.
HDF               Specific settings of the space groups can be
identified either by
HDF               their Hall symbol or by specifying their symmetry
operations.



> data_space_group_symop_operation_xyz


>                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.

HDF ****
  The phrasing in this paragraph leaves me perplexed. After 15 minutes,
I'm still not sure exactly what is intended. My problems: 
  " ... all the operations of the space group "  This is an infinite set
so the list one types in, definitely does not contain all the operations
of the space group. One types in a finite list which represents the
complete set of operations of the space group. One types in a list of
representatives. There are two obvious ways of undertaking the
decomposition which leads to the finite set of representatives.
 (a) With respect to the set(group) of ALL the lattice translations
expressed on the chosen basis (i.e. all primitive translations and all
non-primitive translations)
 (b) With respect to the set(group) of the primitive lattice
translations expressed on the chosen basis. 
The trouble is obviously with cells that are centred in the conventional
choice of basis. ITA does not list out the complete set of
representatives for decomposition (b). What is presented is a coded
manner of being able to generate the representatives for both
decompositions (a) and (b). For (a) you type in the list of coordinates
given for the general symmetry-equivalent position [and ignore the line
giving the centring translations], and for (b) you have to add in the
centring translations to each of the coordinates of the general
equivalent position. I'm not sure what is intended exactly. The
paragraph needs rewriting so that I can understand it. 

So, although I note:

># COMMENT: 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.  Some changes have been made in the descriptions
># and examples to make them more appropriate for the core dictionary.
>#
># STATUS: Preliminary approval 2003-03-11

for me that paragraph needs rewriting either as:


When a list of symmetry operations is given, it must contain a complete
set of coordinate representatives which generates all the operations of
the space group by the addition of all the pure translations of the
space group, both primitive and non-primitive. Such representatives are
to be found as the coordinates of the general-equivalent position in
International Tables for Crystallography Vol. A. to which it is NOT
necessary to add any centring translations.

or


When a list of symmetry operations is given, it must contain a complete
set of coordinate representatives which generates all the operations of
the space group by the addition of all primitive translations of the
space group. Such representatives are to be found as the coordinates of
the general-equivalent position in International Tables for
Crystallography Vol. A. to which it is absolutely necessary to add any
centring translations shown above the general-equivalent position.

End of HDF ****

Back to writing my contribution to Indaba IV. Could you believe it, I've
already done all of the slides as a powerpoint presentation.

H.

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