Items for the coreCIF dictionary relating to restraints
George M. Sheldrick
gsheldr at shelx.uni-ac.gwdg.de
Tue Jan 22 08:55:28 GMT 2008
Dear David,
The proposed list looks very sensible to me and Ilia has done an excellent
job of sorting it out. I like the idea of using an esd of zero to
distinguish constraints from restraints.
I may be mistaken, but as far as I can see, the probably most commonly
applied constraint in SHELXL (and probably other programs too) is still
not covered. I refer to the case where there are disordered solvent
molecules or side-chains, where one GROUP of atoms (e.g. a solvent
molecule) have the same refinable occupancy p and the atoms in the
alternative groupall have occupancies 1-p. This is a powerful constraint
because only one extra parameter p can describe the occupancies of a
large number of atoms. Maybe there is a way of using 'restraint 10' for
this but it is not clear to me, and such a common case should be covered
explicitely.
Without a concept that maps onto the SHELX 'free variables' it will
never be possible to produce a CIF file that enables a refinement to be
repeated. In my personal opinion this is the primary reason for wanting
to encode the restraints and constraints in CIF, without this it is a
rather academic exercise.
I am getting old and senile and in addition to plenty of teaching and
administration have a long list of more exciting things to program -
primarily in macromolecular crystallography - almost certainly more
than I will ever manage to achieve - so I have no plans to adapt
SHELXL to produce these CIF items, even if as seems likely I release
further updates of this program. I have not forgotten what a torture
it was to create and read CIFs before proper tools were available,
but it probably made the difference in getting CIF generally accepted
by small molecule crystallographers. Maybe someone else will have
time to produce software that reads a SHELXL .res file and writes the
restraints and constraints to a CIF file.
Best wishes, George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-2582
On Thu, 13 Dec 2007, David Brown wrote:
> Dear Colleagues,
>
> Ilia Guzei and I have put together the attached document describing a way in
> which constraints and restraints applied during a structure refinement could
> be recorded in a CIF. There has been some demand for such an addition to the
> core dictionary.
>
> I would appreciate it if you could look over the attanched document and let me
> have any comments before Jan 20. The proposal is in the form of CIF examples
> as this should be easier to read and understand than reading dictionary code.
> We will construct the dictionary code after we have your comments. You will
> have a chance to check the code before the dictionary is submitted for final
> COMCIFS approval.
>
> PLEASE MAY I HAVE YOUR COMMENTS BEFORE 20 JANUARY 2008. Let me know if you
> need an extension.
>
> I look forward to hearing from you.
>
> David
>
>
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