From:	windus -x- at -x- mercury.chem.nwu.edu (Theresa Windus)
Date:	Tue, 26 Apr 1994 10:58:48 -0500 (CDT)
Subject:	Re: CCL:Use of symmetry in optimisations

In reply to Patrick Bultinck's question about symmetry:
>
> In section 4 of the GAMESS manual, I find under a title : ***The role of
> symmetry*** that it is best to use symmetry during an optimisation.
> However GAMESS never lowers the symmetry during an optimisation. It is a
> well known fact that sometimes a minimal energy structure differs (a lot)
> from the symmetry you might expect. Isn't it then safer to always do
> unconstrained optimisations, and if you think that the result differs so
> little from a certain symmetry point group, you can always do a
> symmetry-optimisation starting from the geometry with that symmetry, and
> compare energies...
>
> The most important disadvantage is off course that it often takes a lot
> of time to reach a certain symmetry point group, when you start from
> zero-symmery.
>
  Several quantum chemistry codes do not lower symmetry during an
optimization.  As a matter of fact, if the gradient does not reflect
the full symmetry of the point group used, there is a symmetrization
problem.
  Concerning using point group symmetry or not:  If a molecule has
a possibility of a higher point group symmetry than C1, it is a
good idea to exploit it.  Integrals, optimizations, etc. can run
faster if the program is able to use symmetry information.  While
the molecule with symmetry may not be the global minimum on the
surface it is certainly some sort of stationary point on the
surface.  (Note that it might be a transition state or "higher order
transition state".  If the stationary point is a transition state, it
is possible to follow the imaginary frequency leading to a minimum.
You would also have found an important point on the reaction surface!)
  Even if you find a non-symmetric minimum that is lower than the symmetric
molecule, you are not guaranteed that it is the global minimum.  Only in
fairly small molecules is it "easy" to verify that the global minimum
has been found.
  As was pointed out, it can take time to "find" symmetry if you
don't start out with it.  On the other hand, if you have a symmetric
molecule, it is possible to distort it into lower symmetry and
search for a lower symmetry molecule.  My point is that even if it
is not necessarily the lowest energy isomer on the surface, a
molecule with symmetry may tell you something about the surface and
it is a good starting point.  Of course, "forcing" a molecule into
higher symmetry than is practical is not necessarily a good idea
either.
  Hope this helps,

Theresa Windus
Department of Chemistry        e-mail: windus' at \`chem.nwu.edu
Northwestern University


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