Fast track proposal for an addition to the Core CIF dictionary

[David Brown]

Dear Colleagues,

COMCIFS is developing a process for fast tracking minor dictionary 
changes, instead of letting them accummulate until there are enough for 
a major revision.  The process proposed is that, when a request for an 
uncontrovesial new data item is received, it should be directly 
forwarded to the appropriate Dictionary Maintenance Group (DMG) for 
discussion and, if appropriate, approval.  On approval by the DMG it 
would be posted on the COMCIFS web site for a defined period of around 
six weeks.  If at the end of that time, it had raised no comments, or if 
all queries had been resolved, the change would be deemed to be approved.

In the interests of expedition, I am proposing to use the same approval 
method in the core DMG, namely I will post the suggested items for six 
weeks for comment and assume that if you fail to comment it means you 
approve.  Thus if you believe there is a problem with the new item, the 
onus is on you to reply to the core discussion list within the six week 
period.  Once approved by the DMG, the proposal will not come back later 
for a final approval (as we have done with more complex changes) so I 
need to hear from you within the six week period.  If I receive no 
comments I will pass the proposal to COMCIFS for formal approval.

I have received from James Hester the following proposal for a new data 
item and a consequent modification to another item.  I am requesting any 
comments be posted before the end of September.  Assuming that there are 
no unresolved issues at that time I will then pass the proposal to 
COMCIFS for their approval,

David Brown
Core DMG Chair

 > The following is a draft version of a proposed
 > _diffrn_radiation_wavelength_nominal data tag for the current coreCIF
 > dictionary, together with an alteration to the
 > _diffrn_radiation_wavelength description as well. 
 >
 > Request for inclusion of new core CIF item:
 > _diffrn_radiation_wavelength_nominal
 > ============================================================
 >
 > Justification: The wavelength used for a measurement defines the
 > length scales used in the crystallographic unit cell, so it is
 > important that the quality of the number used for wavelength is
 > understood.  When the bulk of experiments came from lab X-ray sources,
 > the wavelength value was an accurate and precise quantity that could
 > be relied on.  In contrast, synchrotron and neutron sources have
 > comparatively poorly-defined wavelengths, subject to systematic errors
 > (for example, changes in the monochromator d-spacing due to
 > temperature effects, and angular offset errors).  Many facilities do
 > not attempt to characterise the accuracy of such derived wavelength
 > values. Instead users may (or may not) refine against a standard
 > compound in order to recover an accurate wavelength value. I therefore
 > propose a separate tag for those wavelength values that are nominal,
 > that is, derived from positioning of optical elements rather than
 > proper standards.  This removes ambiguity in determining whether or
 > not a wavelength has been obtained from refinement against a standard,
 > or is simply a nominal value based on beamline optics.
 >
 >
 > DDL1 definition
 >
 > data_diffrn_radiation_wavelength_nominal
 >     _name                      '_diffrn_radiation_wavelength_nominal'
 >     _category                    diffrn_radiation_wavelength
 >     _type                        numb
 >     _list                        both
 >     _list_reference            '_diffrn_radiation_wavelength_id'
 >     _enumeration_range           0.0:
 >     _units                       A
 >     _units_detail              'angstroms'
 >     _definition
 > ;              The incident radiation wavelength in Angstroms
 > calculated from secondary information, for example monochromator
 >                angle or time of flight.  If the wavelength has been
 > determined using some type of standard, _diffrn_radiation_wavelength
 > should be used
 > ;
 >
 > data_diffrn_radiation_wavelength
 >     _name                      '_diffrn_radiation_wavelength'
 >     _category                    diffrn_radiation_wavelength
 >     _type                        numb
 >     _list                        both
 >     _list_reference            '_diffrn_radiation_wavelength_id'
 >     _enumeration_range           0.0:
 >     _units                       A
 >     _units_detail              'angstroms'
 >     _definition
 > ;              The radiation wavelength in angstroms as determined
 > from measurements using standards, for example an
 >                X-ray emission line, or when a refinement based on a 
standard
 >                material has been carried out.  The details of such a
 > refinement should be recorded in the _diffrn_radiation_special_details
 > item.
 > ;

The only change in the latter item is in the defiition.  The current 
definition reads 'The radiation wavelength in angstroms'.

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