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Re: Printing of multiple geometry values

  • To: Multiple recipients of list <coredmg@iucr.org>
  • Subject: Re: Printing of multiple geometry values
  • From: "I. David Brown" <idbrown@mcmail.cis.McMaster.CA>
  • Date: Thu, 3 Sep 1998 13:33:42 +0100 (BST)
	While Acta Cryst may wish to keep the flexibility associated with
use of a non-standard table, the truth is that in most cases where there
are multiple bonds, i.e. in the example given by Brian, one does not need
the symmetry operators, the bonding is usually quite obvious without them.
Thus I see the use of _geom_bond_mult as an alternative to giving the
bonds with symmetry operators rather than a way of shortening the length
of a cif.  Thus an author could either list all the bonds with their
symmetry operators, or give a list of symmetry independent bonds with a
multiplier.

	How Acta Cryst. uses this new feature is up to them - they could
carry on as they always have, but an alternative would be to allow authors
reporting high symmetry structures to give multiplicities rather than
symmetry operators.  There are cases in high symmetry structures where,
for lack of suitable cif-writing software, authors have to enter bond
lengths by hand.  This entails working out and entering all the symmetry
operations which, in the event, are never printed.  Seems like unnecessary
work, particularly as anyone really interested will recalculate the bonds
from the retrieved cif, rather than rely on the author's list.

	If one wants to list only the symmetry unique bond lengths
together with a multiplicity and then list all the operators that generate
equivalent bonds, a subsidiary loop linked back through a _geom_bond_id
would be needed, but this would be overkill.

	I suppose while we are about it we should also define
_geom_angle_mult.  I do not see that _geom_torsion_mult makes any
sense. 

			David


*****************************************************
Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
idbrown@mcmaster.ca
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