CCL:re DFT and small basis sets

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
    Royal Society University Research Fellow
    Chemistry Department
    University College London  phone:  +44 (0)20-7679-4663
    20 Gordon Street          e-mail: work: T.vanMourik.-at-.ucl.ac.uk
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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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