Re: CCL:symmetry in electronic structure computations

[Dan Severance <dan -x- at -x- omega.chem.yale.edu>]

    Hi,
       Just a few more remarks regarding Richard Bone's comments:
 > "Gaussian" finds symmetry present in the input nuclear
 coordinates and
 > applies it unless you explicitly switch it off.  But geometry optimizations
 > will typically, irritatingly, quit as soon as a change in point group
 occurs.
       If you wish to force the symmetry, this can be averted by using the
 same variable name for the symmetric lengths and angles, thus there is
 no possibility of a change in point group.
 > The gradient of the energy w.r.t. nuclear displacements should transform as
 > the totally symmetric 'irrep' in whatever point group you happen to be in.
 >For 'closed-shell' systems (no electronic degeneracies) this means in
 practice
 > that 'symmetric' structures are usually stationary points.  The only
       There are actually a number of cases where this is indeed not the
   case.  Amides, for instance, tend not to be planar but are slightly
   puckered at the N.  It often doesn't take much energy to force it to
   be planar, but non-planar is the minimum at any levels of theory that
   I've seen.  Also, quite bulky molecules often avoid a symmetric
   form in favor of spreading out the steric interactions.
       The only way to be sure anything is really a minimum or a
   transition state is to perform a frequency calculation at the end,
   whether or not one uses symmetry in the calculation.  I had started
   the methyl groups in DMA in one orientation and they stayed put, even
   though I did not invoke symmetry.  The frequency calculation showed
   it to be a transition state, and further optimizations using those
   forces lead to the minimum structure.
       The use of symmetry is still quite useful in speeding
   computations, and the freq. calc. will tell you whether you were
   correct or incorrect in assuming the particular level of symmetry.
   (Use individual variables for each bond and angle if you want to
   optimize further, though!)
      Dan Severance  (APLS - Association for Prevention of Long Sigs)
      dan -x- at -x- omega.chem.yale.edu