CCL:G: DFT and dispersion

 Sent to CCL by: Arne Dieckmann [adieckma()googlemail.com]
 Dear all,
 I am currently dealing with structures which require to compensate for the
 missing dispersion corrections in DFT. Therefore, I have optimized all
 geometries with M062X/6-31+G(d,p) as this functional already takes into account
 medium-ranged dispersion interactions (I know that a larger basis set would be
 desirable, but the systems size prevents this). However, my suspicion is that
 also long-ranged dispersion effects are not negligible for my structures. As a
 consequence, I would like to apply some kind of DFT-DX (X=2, 3) correction by
 Grimme. Can you tell by your experience if DFT-D2 would change M06-2X geometries
 significantly or if an additional single point correction to the energy is
 sufficient? Does it really make sense to add DFT-D3 corrections to M062X
 energies? Which other functional should I use (B97-D?)? Please note that I am
 limited to using Gaussian 09 which means I can only use DFT-D2 for
 optimizations.
 Cheers,
 Arne
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 Dr. Arne Dieckmann
 Houk Research Lab
 University of California, Los Angeles
 email: adieckma#googlemail.com