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RE: twinning definitions for CIF files

• Subject: RE: twinning definitions for CIF files
• From: "Jeffrey Deschamps" <[email protected]>
• Date: Fri, 29 Jun 2001 20:05:15 +0100 (BST)

> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of David Watkin
> Sent: Friday, June 29, 2001 4:14 AM
> To: [email protected]
> Subject: Re: twinning definitions for cif files
> 
> 
> Hello
> 	We use an alternative program which is quite smart at refining
> twins, and so would be a little upset if a standard was set which was
> SHELX-specific. 
> 
> Quoted text isn't too friendly for the programmer trying to unpick a cif,
> but it does provide an unbiased mechanism for recording the information
> until some standards are agreed. It seems a good interim solution.

If quoted text isn't the answer (and I agree that it is NOT the long-term
answer) then we need to begin discussing what should be in the CIF file.
That was the original question.

For starters I think we need a field in the CIF such as:

	_refine_twin_type

that can 'values' of none (the same as a null field), 'merohedral',
or 'non-merohedral'.  I'm not sure what the best place is for this
information (i.e. _refine, _exptl, or _diffrn).   I'm sure the
experts on the CIF dictionary can come up with this.

Other parameters would result from the different types of twins.  For 
a merohedral twin you would also want to know the twin operation that 
relates the two (or more) orientations, and the fractional occupation 
of each orientation.  I think the non-merohedral case will be harder to 
deal with as you are likely to have some additional information in with 
the structure facors.  The software used to refine the twin should also
be reported, esspecially if it required some preprocessing to produce a
scaled HKL file whch inclueds information of the various twins.

A much more elgant description of how this could be handled was posted to
the Structural Chemistry discussion list by Brian McMahon.  That list can 
be found at:

	http://iucr.sdsc.edu/iucr-top/lists/strchem/


Jeff

__________________________________________
Jeffrey Deschamps, Ph.D.
Code 6030
Naval Research Laboratory
Washington, DC 20375
(202)767-0656    FAX:(202)767-6874 

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