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Re: Draft changes to Core data names for multiplicity etc.

I am not sure if I am "in" or just a kibbitzer. but this sounds sensible 
to me. -- Herbert

On 5/16/12 6:32 AM, Brian McMahon wrote:
> 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:  bm@iucr.org
> 5 Abbey Square, Chester CH1 2HU, England
> _______________________________________________
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> coreDMG@iucr.org
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>
>    

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