Subject: one more symmetry argument

From: "Tang, Huang (UH-Chem)" <[email-masked]>
Date: Fri, 29 Apr 1994 12:12:44 -0500 (CDT)

<UH-Chemistry>, 29-APR-1994

Dear Netters,

We have seen lots of symmetry related messages.  It seems to me there are
some confusions regarding using symmetry in calcualtions.  IMHO, to use
symmetry or not is totally intuitively-based decision.  There is no a priori
reason to prove that symmetric or asymmetric geometry should be minimum or
not.  But chemical sense (VSEPR rules based) always favors to start from
symmetric geom.  Take TiCl4 (or simply CH4) as an example, it makes little
sense to start from C1 symmetry, though you'll get to Td geom eventually and
costly.  As molecules are bigger and more complicated, using symmetry is
always a good starting point.  Though high symmetry geom might not be
minimum, it also gives us certain info anout the structure. The example is
MoH6 which is not Oh structure!

For some multi-well potential surfaces, even you start from C1 symmetry,
there is no guarrantee you'll reach a global minimum.  Therefore using lower
symmetry is not a good starting point, unless your intuition tells you to do
so (how from the beginning?  I don't know :-)

There are some confusions about minimum (saddle point) and stationary point.
We all refer these terms to OPTIMIZED structure, not randomly drawn
structure.  Using any symmetry constraint, you can always get to a
stationary point (counter-examples? I'd like to hear one :-), meaning you
can always optimize it to a point where all gradients vanish (within a given
threshold).  But the stationary point may not be a minimum.  At times you
have to break or lower symmetry to reach a minimum (after you are sure
from a freq. calcn.)

All in all, computational chemistry is still chemistry.  Do whatever makes
chemical sense. 

Happy computing...

--------                
   _         Tang, Huang
 _| ~-       Chem. Dept, U. of Houston
 \,  _}      Houston, Texas 77204-5641
   \(        e-mail:  [email-masked]
             phone:   (O)  (713) 743-3269