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Re: New fast-track definition proposal:_diffrn_radiation_wavelength_nominal

Here is the problem I have with this approach. It seems to me that you are
suggesting there are two wavelength values, both of which are relevant to
the data set and both of which have to be recorded.

But what I think you have written is that there is only one wavelength, but
you are trying to record something about its properties. I can appreciate
the need for a data item that indicates something about the properties, but
don't appreciate the need for a new wavelength data item.

Perhaps something like

_diffrn_radiation_wavelength

and

_diffrn_radiation_wavelength_determination

The latter being free text or preferably an enumerated type that indicates
the various possibilities. And one could re-use

_diffrn_radiation_special_details

to provide all additional information.

I would be very wary of VARIANTS as discussed by Herb in a follow up
discussion. Adding variations to existing definitions is going to be a great
source of error from users. At its basis is a desire to employ CIF in a way
it was not originally intended to be used. CIF is essentially an archival
format, for the submission of the final model/data/results etc. Herb is
suggesting its use as a lab manual to record every assumption, through
process, error etc during the experiment and refinement. Vary admirable, but
not what CIF is about, and probably such a CIF would not be of great use
outside of the home lab.

The new DDLm etc supports many powerful features for one to make local
changes to dictionaries, and while I have no problem with Herb's approach to
meet local needs (apart from the fact I would do it differently), I would be
concerned if it was part of the official archive. The import feature of DDLm
allows one to make changes at many levels of refinement to the dictionary -
preferably for local use. The official IUCr submission dictionary (for
archive purposes) will have different requirements.


On 3/11/09 10:52 AM, "James Hester" <jamesrhester@gmail.com> wrote:

> Dear COMCIFS members: the following fast track proposal for a minor
> change to the coreCIF dictionary has been approved by the core
> Dictionary Maintenance Group after a 6-week comment period.  As per
> the recently approved fast track process, it is now open for comments
> from COMCIFS for a further 6 week period.
> 
> James.
> ------------------------------------------------------------------------------
> ----------------
> 
> 2009-08-23 proposed by James Hester.
> 2009-10-16 approved by the core Dictionary Maintenance Group under a fast
>           track approval process.
> 
> Explanation
> ===========
> 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.
> 
> 
> A new DDL1 item
> 
>  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
> ;
> 
> A change in definition of an existing item.  The current
> definition reads 'The radiation wavelength in angstroms'.
> 
> 
>  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.
>  ;

cheers

Nick

--------------------------------
Associate Professor N. Spadaccini, PhD
School of Computer Science & Software Engineering

The University of Western Australia    t: +61 (0)8 6488 3452
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