| From: |
"Chuck Ulmer, D.A.Smith Group" <CULMER -8 at 8- uoft02.utoled |
| Date: |
Tue, 26 Apr 1994 12:32:25 -0500 (EST) |
| Subject: |
Re: Symmetry |
Patrick Bultinck writes.....
>Netters,
>
>I have just sent a reply to a question concerning the use of a Z-matrix
>for a calculation on ethylene, using GAMESS (US).
>
>As a direct consequence I rembered something that I am not very
>comfortable with...
>
>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.
>
>Any thoughts on this...
Yes, I would like to see a discussion on this subject opened up. I would
especially be interested in comments from some of the more experienced
computation chemists. Also, those persons who did calculations long enough
ago when the lack of computer power necessitated the use of symmetry could,
I am sure, have some interesting input on this subject.
Up to this point, I thought that the use of symmetry in calculations was
a big NO-NO because you are arbitrarily constraining the system and "garbage
in is garbage out". But I just ran across a time when we had to use symmetry:
We are investigating a complexation between a small organic molecule and a
fragment containing several metal centers. The whole point is to match the
metal orbitals with those calculated for the organic fragment and determine
why one fragment reacts one way and other fragments react anther way. The
only way that we can make sense out of the orbitals in the organic fragment
is to constrain the system to some point group - must get orbital symmetries.
So, the questions is: If this works, is it by some stroke of luck or does
this reaction ONLY occure if the organic fragment (considering free rotation)
reaches d3h symmetry while in the proximity of the metal centers? (I am being
facetious to some degree).
Don't we, as scientists, have to keep all constraints and limitations of
our model in mind when looking at systems and use our chemical intuition as
a guide? Isn't the use of symmetry just one more constraint to keep in mind
or does the use of symmetry *invalidate* any scientific findings?
Of course, the symmetry argument (to use or not to use) probably goes along
with the "bigger is better" argument and I will probably still stay away
from symmetry as much as possible and run with the largest basis set and
highest level of theory that disk allows, provided OSC permits.
Chuck
--
Charles W. Ulmer, II
Graduate Student Senior Scientist
D.A.Smith Group DASGroup, Inc.
University of Toledo 3807 Elmhurst Road
Toledo, OH, 43606 Toledo, OH 43613
phone: (419)537-4028 culmer &$at$& uoft02.utoledo.edu
fax: (419)537-4033
WE ARE PERHAPS NOT FAR REMOVED FROM THE TIME WHEN WE SHALL BE ABLE TO
SUBMIT THE BULK OF CHEMICAL PHENOMENA TO CALCULATION.
-- JOSEPH LOUIS GAY-LUSSAC
MEMOIRES DE LA SOCIETE D'ARCUEIL, 2, 207 (1808)
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