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Draft changes to Core data names for multiplicity etc.

Dear Colleagues

This message is being sent to members of the CIF Core Dictionary
Management Group, who are requested to respond to it at your
earliest convenience. A copy will also be sent to other software
developers for their early information.

Some programs have output values for the core data item
_atom_site_symmetry_multiplicity that are actually the order
of the space-group site symmetry and not the multiplicity as
defined in International Tables. As a result, the values given
in CIFs in the IUCr archive and elsewhere are ambiguous.

It is proposed to remove this ambiguity by deprecating the
existing data name _atom_site_symmetry_multiplicity and replacing it
with two new names, 
    _atom_site_site_symmetry_multiplicity
    _atom_site_site_symmetry_order
formal definitions of which are given below.

This will allow future "remediation" of the IUCr CIF archive to
remove this ambiguity; and software authors may cleanly update their
output in the next release cycle of their programs. In practice,
we think that the usage of existing CIFs will not be greatly affected,
since most programs do not import and use the site multiplicities
directly; but by making this formal change we will also be drawing
attention to the need for end-users who do rely on this value to
validate it before use.

Please let us know whether you agree to, or have any objections to
this approach, as soon as possible.

For your information, it is proposed also to make two small editorial
changes to other data items (clarifying that F(000) may be given 
in the case of neutron diffraction experiments, and sanctioning the
use of standard uncertainties when quoting the wavelength, as is
practice in powder diffraction). The details are also included below.

Best wishes
Brian McMahon
COMCIFS Coordinating Secretary

-------------------------------------------------------------------

    _dictionary_history
;
   (extract)
   2012-05-15 BMcM: Deprecated _atom_site_symmetry_multiplicity;
                    replaced with _atom_site_site_symmetry_multiplicity
                    and added _atom_site_site_symmetry_order (IDB)
               Reworded _exptl_crystal_F_000 definition to take
                        account of neutron diffraction and removed
                        _enumeration_range (can be negative for neutrons)
               Added '_type_conditions su' to _diffrn_radiation_wavelength
;

data_atom_site_symmetry_multiplicity
    _name                      '_atom_site_symmetry_multiplicity'
    _category                    atom_site
    _type                        numb
    _list                        yes
    _list_reference            '_atom_site_label'
    _related_item              '_atom_site_site_symmetry_multiplicity'
    _related_function            replace
    _enumeration_range           1:192
    _definition
;              The multiplicity of a site due to the space-group symmetry as
              given in International Tables for Crystallography Vol. A (2002).

               Use of this data name is deprecated because of
               inconsistencies in practice among structure refinement
               software packages. The number of positions given for
               this Wyckoff site in International Tables for
               Crystallography Vol. A (2002). should now be expressed
               using the data name _atom_site_site_symmetry_multiplicity.
               In the historic archive some CIFs use this item to give values
               that belong in _atom_site_site_symmetry_order.
;

data_atom_site_site_symmetry_multiplicity
    _name                      '_atom_site_site_symmetry_multiplicity'
    _category                    atom_site
    _type                        numb
    _list                        yes
    _list_reference            '_atom_site_label'
    _related_item              '_atom_site_symmetry_multiplicity'
    _related_function            alternate
    _enumeration_range           1:192
    _definition
;              The number of different sites that are generated by the
               application of the space-group symmetry to the
               coordinates given for this site. It is equal to the
               multiplicity given for this Wyckoff site
               in International Tables for Crystallography Vol. A (2002).
               It is equal to the multiplicity of the general position
               divided by the order of the site symmetry given in
               _atom_site_site_symmetry_order.
;

data_atom_site_site_symmetry_order
    _name                      '_atom_site_site_symmetry_order'
    _category                    atom_site
    _type                        numb
    _list                        yes
    _list_reference            '_atom_site_label'
    _enumeration_range           1:48
    _definition
;              The order of the site symmetry of the site identified by
               _atom_site_label.

               This is the number of times application of the
               crystallographic symmetry to the coordinates given for
               this site generates the same set of coordinates.
               It is equal to:

                          multiplicity of the general position
                          ------------------------------------
                              multiplicity of this site

                where 'multiplicity of this site' is
                given in _atom_site_site_symmetry_multiplicity.
;

data_diffrn_radiation_wavelength
    _name                      '_diffrn_radiation_wavelength'
    _category                    diffrn_radiation_wavelength
    _type                        numb
    _type_conditions             su        
    _list                        both
    _list_reference            '_diffrn_radiation_wavelength_id'
    _enumeration_range           0.0:
    _units                       A
    _units_detail              'angstroms'
    _definition
;              The radiation wavelength in angstroms.
;

data_exptl_crystal_F_000
    _name                      '_exptl_crystal_F_000'
    _category                    exptl_crystal
    _type                        numb
    _list                        both
    _list_reference            '_exptl_crystal_id'
    _definition
;              The expression for a structure factor evaluated in the
               zeroth-order case h = k = l = 0, F(000). This may contain
               dispersion contributions and is calculated as

               F(000) = [ (sum f~r~)^2^ + (sum f~i~)^2^ ]^1/2^

               f~r~   = real part of the scattering factors at theta = 0
               f~i~   = imaginary part of the scattering factors at theta = 0

                        the sum is taken over each atom in the unit cell

               For X-rays, non-dispersive F(000) is a positive number
               and counts the effective number of electrons in the unit cell;
               for neutrons, non-dispersive F(000) (which may be negative)
               counts the total nuclear scattering power in the unit cell. See
                  http://reference.iucr.org/dictionary/F(000)
;

------------------------------------------------------------------------------
Brian McMahon                                       tel: +44 1244 342878
Research and Development Officer                    fax: +44 1244 314888
International Union of Crystallography            e-mail:  [email protected]
5 Abbey Square, Chester CH1 2HU, England
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