[CHM 331] Geometry Optimization Strategy

[Andrew J. Pounds]

  At least two of you have approached me about optimization problems 
(Gaussian taking a LONG time to optimize a structure).  Here is a 
strategy that I generally follow to speed up this process.   Since most 
of you are optimizing normal molecules (not trying to do bizarre things 
like force atoms to be close to each other) then I recommend the following.

   1. Build the basic molecule backbone and then add the functional
      groups.  Make sure that no atoms are too close to each other.  You
      can easily do this now by chaning the angles and dihedrals for
      individual atoms.
   2. Optimize using a Semiempirical technique (I usually selecte PM3). 
      In most case this will put the molecule in a confirmation that is
      near the minimum.
   3. With the PM3 optimized structure start doing your "real"
      optimization using either HF/DFT or MP2.  As many of you saw in
      P-Chem -- you are going to need to put polarization functions on
      those basis sets.  In other words, do you optimizations with at
      least 3-21G* and, it you have time, 6-31G(d,p)

If you are still having issues getting molecules to optimize, send me 
the .gjf file and I'll see what I can do.


-- 
Andrew J. Pounds, Ph.D.  (pounds at theochem.mercer.edu)
Associate Professor of Chemistry and Computer Science
Mercer University,  Macon, GA 31207   (478) 301-5627

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