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Re: coreCIFchem #5

  • To: corecifchem@iucr.org
  • Subject: Re: coreCIFchem #5
  • From: Howard Flack <crystal@flack.ch>
  • Date: Mon, 09 Aug 2004 17:52:04 +0200
After e-mail correspondence between John Rutherford, David and myself, 
in my TNT cif replace:

 > _tecton_symmetry    # point group if geometry present otherwise 

by two items (the table of values also needs modifying)

_tecton_geometric_class  #
_tecton_graph_automorphism_group #

_tecton_geometric_class is the orientation-independent specification of 
the tecton point group according to its geometry. It's probably best to 
use a Schoenflies symbol as there is no choice of basis implicit in the 
geometry as given by interatomic distances, interatomic (dihedral) 
angles. I suppose that if the geometry is given as a set of coordinates, 
it might be justifiable to specify a point group with a H-M point group 
symbol for the tecton orientation corresponding to the atomic coordinates.

_tecton_graph_automorphism_group encodes the symmetry of the graph as a 
group of permutations (of atoms). If I understand correctly there are no 
standard symbols for these automorphism groups although it seems that in 
a fair number of cases they are isomorphic to a point group in three 
dimensions. So often one could use a Schoenflies symbol. [Even that is 
equivocal - point groups Cs, Ci and C2 are isomorphic] The TNT graph 
should be given the symbol C2v for its _tecton_graph_automorphism_group. 
[C2v is isomorphic to D2 and C2h. Why is it I prefer C2v? Am I (are we) 
too geometry oriented? I think we (David?) need to call John Rutherford 
again.] What one does in the case that the graph automorphism group is 
not isomorphic to a 3D point group, I do not have the least idea apart 
from writing a ? .


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