Subject: re DFT and small basis sets

From: elewars <elewars-.at.-trentu.ca>
Date: Wed, 02 Jul 2003 10:31:57 -0400

2003 July 1

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.
>