Subject: CCL:re DFT and small basis sets

From: Tanja van Mourik <t.vanmourik.-at-.ucl.ac.uk>
Date: Thu, 3 Jul 2003 11:58:23 +0100 (BST)

Hi All,

Lynch, Zhao and Truhlar advocate including diffuse functions on
non-hydrogenic atoms when calculating reaction energies, barrier 
heights and conformational energies with DFT, see:
J Phys Chem A 107 (9): 1384-1388 (2003).

So I would recommend using 6-31+G* rather than 6-31G* as the smallest
basis set for yielding accurate results.

Tanja
-- 
  =====================================================================
   Tanja van Mourik                                                
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   Chemistry Department 
   University College London  phone:  +44 (0)20-7679-4663      
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> 
> Re "Has anyone ever published a discussion of the accuracy of DFT at
> small basis sets?"
> 
> I'm not sure just what you consider small (most workers now would likely
> say that
> 6-31G* is small), but DFT calculations are known to be saturated by
> basis
> functions more quickly than are ab initio calculations. Here are three
> refs:
> 
> 1)  "Once the double split-valence level is reached, further improvement
> in basis
> set quality offers little in the way of structutal or energetic
> improvement." G.
> N. Merrill, S. Gronert, and S. R. Kass. J Phys Chem, 1997, _101_, 204.
> 
> 2)  "Our results also show that B3LYP calculations converge rapidly with
> 
> increasing basis set size and that the cost-to-benefit- ratio is optimal
> at the
> 6-31G* basis set level. 6-31G* will be the basis set of choice in B3LYP
> calculations on much larger molecules [than C4H6O2]." P. J. Stephens, F.
> J.
> Devlin, C. F. Chablowski, and M. J. Frisch, J Phys Chem, 1994, _98_,
> 11623, and
> refs therein.
> 
> 3)  In defiance of tradition, the unequivocally small 3-21G(*) set has
> been used
> to optimize carbene geometries: H. M. Muchall, N. H. Werstiuk, and B.
> Chodhurry,
> Can J Chem, 1998, _76_, 221.
> 
> (You my also find some useful info in E. Lewars, "Computational
> Chemistry",
> Kluwer, 2003; section 7.3.2.2).
> 
> E. Lewars
> ========
> 
> Ed Brothers wrote:
> 
> > Folks:
> >     Has anyone ever published a discussion of the accuracy of DFT at
> > small basis sets?
> >
> > Ed.
> > Merz group.
> > Penn State.
> >
> 
> 
> 
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