Subject: gauche effect ab initio

From: [email-masked] (Dawn Friedman)
Date: Wed, 1 Jun 94 15:49:33 -0400

Use the best basis set you can, with plenty of polarization functions,
and diffuse functions (+) would be good too, as you will be looking at
systems whose resonance structures would include lots of lone pairs and
possibly anions; 3-21G*+ at the minimum, but preferably 6-31G**++.  

Use the
best basis set you can muster for the geometry optimization; energies
calculated at that geometry but at a higher basis set probably won't give
you much more information than you already have.  If you don't reoptimize
the geometry at each dihedral angle you try, your results will be deceptive,
but everyone knows that by now, right?
  
You'll want to do some post-Hartree-Fock, perhaps CISD; if you really
need to try for the absolute geometry changes between conformations, 
you'll have to
try optimizing with post-HF, but HF will give you the trends in geometry
changes.
  
If you're using Gaussian, which has it available as a keyword, the
Reed-Weinhold Natural Bond Orbitals are custom-made for this sort of thing.
They'll tell you which orbitals are interacting and how much (relative
to other systems in the set you're looking at.)
  
I'd be interested to hear about your results; this is an old pet problem
of mine.
  
  --Dawn Friedman   [email-masked]