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                          F I N A L     P R O G R A M

                          PSEUDOSYMMETRY SESSION 1994
                 (Sponsored by the Service Crystallography SIG)

                              Organizing Committee
                      Patrice W. Rafalko (Murray Hill, NJ)
                         Douglas M. Ho (Princeton, NJ)


10:00  SOLVING THE PHASE PROBLEM FOR STRUCTURES WITH PSEUDOSYMMETRY.
       A PHASE PROBLEM OR A REFINEMENT PROBLEM?  Paul T. Beurskens

10:20  FURTHER STUDIES OF APPROXIMATE SYMMETRY IN CRYSTAL STRUCTURES
       WITH Z' > 1.  Raymond E. Davis, Ning-Leh Chang and Kraig A. Wheeler

10:40  EUCLIDEAN ORBIFOLDS AND THEIR SINGULAR SETS.  Carroll K. Johnson
       and Michael N. Burnett

11:00  SYMMETRY VS PSEUDOSYMMETRY:  CLUES FROM ANISOTROPIC DISPLACEMENT
       PARAMETERS.  Carolyn P. Brock

11:20  Coffee Break

11:40  PSEUDOSYMMETRY AND MULTIPLE ASYMMETRIC UNITS IN STEROID CRYSTAL
       STRUCTURES.  William L. Duax and Jane F. Griffin

12:00  SYMMETRY AND PACKING PROBLEMS IN ORGANOMETALLIC CRYSTALS.
       A SPECIAL CASE OF PSEUDOSYMMETRY.  John C. Huffman

12:20  INTERPRETATION OF SUBTLE DISTORTIONS IN SIMPLE INORGANIC OXIDES.
       Richard L. Harlow

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                       Summary for ACA Newsletter
                        --------------------------

Session Title:   Pseudosymmetry

Day/Date/Time:   Friday, July 1, 1994, 9:30 am - 1:00 pm

Chairperson(s):  Patrice W. Rafalko, BOC Group, Murray Hill, NJ
                 Douglas M. Ho, Princeton University, Princeton, NJ

Session U, Pseudosymmetry was sponsored by the Service Crystallography SIG.
Presentations on structure solution methods, symmetry concepts, and
guidelines for recognizing pseudosymmetry were illustrated with examples
from a wide variety of chemical systems.  All of the talks were well
attended and were followed by lively discussions.

Paul Beurskens served as the lead speaker for the session and presented
an overview of his vector-based PHASEX routine for addressing structure
deviations from specified solution symmetries.  Structure solution methods
under development in his laboratories pertaining to "Solving the Phase
Problem for Structures with Pseudosymmetry - A Phase Problem or a
Refinement Problem?" were described.  He proposed that the latest generation
of solution programs is quite good, and closed by predicting that pseudo-
symmetry is now largely a refinement problem.

Ray Davis (co-authors N.-L. Chang and K. A. Wheeler) followed with a
lecture on space group statistics for structures with multiple molecules
in the asymmetric unit.  Methods for determining fragment or moiety
deviations from an applied symmetry solution set were described and
simple techniques to calculate the magnitude of those deviations were
given.  The approach provided a numeric format for assessing whether
atoms or molecules are related by symmetry or by pseudosymmetry.

Carroll Johnson (co-author M. Burnett) attacked symmetry from a crystal-
lographically nontraditional point of view.  Euclidean orbifolds and
singular sets were constructed and used as a basis for this alternative
topological description of point groups, plane groups, and space groups.
Symmetry and pseudosymmetry being interrelated, this novel provides an
additional method of visualizing the two phenomena.  The utility of
these orbifolds and concepts within our discipline remains to be fully
realized.

Carol Brock recommended that we look critically at atomic displacement
ellipsoids.  Patterns in the displacement parameters can be used to
quickly determine the presence of symmetry or pseudosymmetry as well
as revealing other serious structural problems.  In general, if the
displacement parameters of the multiple molecules in the asymmetric
unit readily transform into each other, it is likely that an element
of symmetry was overlooked and the space group misassigned.  Conversely,
the absence of such a correspondence would argue in favor of pseudo-
symmetry being present.

Bill Duax (co-author J. F. Griffin) pointed out that multiple molecules
in an asymmetric unit are not prohibited from transforming precisely
into one another via a pseudosymmetry operation.  In such cases, the
false conclusion that symmetry may have been overlooked becomes a real
danger.  However, the purported symmetry can be readily discounted by
noting if the related molecules of exist in different crystalline
environments.  His example of the intermolecular hydrogen-bonding
scheme of the steroid dexamethasone clearly illustrated this point.

John Huffman presented an array of organometallic compounds exhibiting
somewhat the opposite phenomenon.  Metal alkoxides were described in
which the ligands were nearly congruent and defined a well ordered,
fixed environment about a disordered core of metal atoms.  Now the
environment is invariant and this implied symmetry is discounted by
the disordered metal pattern within the interior of the molecule.

Dick Harlow ended the session with a discussion of inorganic oxides
and fluorides.  He demonstrated that superlattice reflections arising
from a lower symmetry space group could be easily missed in a routine
single crystal experiment.  To circumvent this, he encouraged us to use
data from a broader range of techniques.  Photographic, neutron, and
synchrotron powder data when available can often resolve these crystal
system and space group conflicts.  Even then, a careful inspection of
subtle distortions, atomic deviations and anisotropic displacement
parameters should necessarily follow.

Pat Rafalko and Doug Ho

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