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Re: Draft CIF dictionary for rgid body description

  • To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group <coredmg@iucr.org>, Ilia Guzei <iguzei@fozzie.chem.wisc.edu>
  • Subject: Re: Draft CIF dictionary for rgid body description
  • From: "George M. Sheldrick" <gsheldr@shelx.uni-ac.gwdg.de>
  • Date: Wed, 11 Aug 2010 18:19:56 +0200 (CEST)
  • In-Reply-To: <4C62BCB9.50009@mcmaster.ca>
  • References: <4C62BCB9.50009@mcmaster.ca>

There is a very good reason why shelxl does not allow an atom to be in 
more than one rigid group: it is mathematically impossible except for the 
special case where the the 'pivot atom' is itself in another rigid group
which defines its x, y and z coordinates. This special case is very
complex to program. Restraints are much simpler to implement and probably 
more effective because they distribute the errors more flexibly.


Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582

On Wed, 11 Aug 2010, David Brown wrote:

> Dear Colleagues,
> In January I circulated to you, the coreCIF Dictionary Maintenance Group, the
> draft of a CIF dictionary containing items for reporting the restraints and
> constraints that were applied during the final round of refinement.  This was
> approved by this group after a six week consultaion period and passed on to
> COMCIFS for final approval.  During this stage an objection was raised that
> under the proposal it was not possible for an atom to belong to more than one
> rigid body, because the rigid bodies were defined by a flag in the atom_site
> list which identified which rigid body a partcular atom belonged to. 
> In order to address this weakness, Ilia Guzei and I have proposed an amendment
> to the earlier draft.  This is more flexible in that it defines each rigid
> body in terms of any atom in the atom_site list together with a symmetry
> operation.  This places no restriction on the number of rigid bodies an atom
> can belong to.  I am circulating this amendment in the attached ASCII file
> which contains the amended dictionary text.
> Can you please check this and respond to this Discussion List with your
> approval or reservation before September 10 (2010-09-10).  If there are no
> unresolved issues at that time, I will incorporate this amendment into the
> original restraint dictionary and pass it back to COMCIFS for final approval.
> I am looking forward to heariing from you
> David Brown
> Chair CoreCIF DMG
coreDMG mailing list

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