Subject: Re: CCL:combination of b3lyp and cc-pVQZ

From: Frank Jensen <[email-masked]>
Date: Tue, 28 Aug 2001 08:36:22 +0200 (METDST)

We have recently done some benchmarking of 
various basis set for DFT. For the H,C,N,O,F molecules
in the G3 data set, the MAD and Maximum error of some
popular basis sets for total atomization energies 
with the BLYP functional are (kJ/mol):

			MAD	Max
	
	6-31G*		16	64
	cc-pVDZ		41	82
	cc-pVTZ		3.6	14.5
	cc-pVQZ		2.9	10.9

	These numbers are errors relative to a saturated basis
set, i.e. the Kohn-Sham limit. The values should be
fairly independent on the functional, including hybrid functionals
like B3LYP. The basis set error should of course be compared 
to a typical error inherently in the functional itself,
which for e.g. B3LYP may be ~20 kJ/mol.
	For relative energies between similar species
the basis set error is likely to be smaller, but we have
no hard data on this yet.

	Frank

On Mon, 27 Aug 2001, elewars wrote:

> 
> Hello,
> 
> DFT calc's seem to be "saturated" by basis sets more quickly than are ab initio:
> Merril et al noted that "Once the double split-valence level [e.g. 6-31G*] is
> reached, further improvement in basis set quality offers little in the way of
> structural or energetic improvement.' [1]. Stephens et al. report 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
> C5H6O2]." [2]. We have found (albeit in a limited number of calculations) only
> very small differences (up to 4 kJ mol-1) between 6-31G* and 6-311G* basis sets
> with the BP86 functional.
> 
> But you could do some test calculations using B3LYP/6-31G* and B3LYP/cc-pVQZ.
> 
> [1] G. N. Merrill, S. Gronert, S. R. Kass, J Phys Chem A, 1997, 101, 208.
> [2] P. J. Stephens, F. J. Devlin, C. F. Chablowski, M. J. Frisch, J Phys Chem
> 1994, 98, 11623, and refs therein.
> 
> E. Lewars
> =========
> 
> 
> Harald Bock wrote:
> 
> > Dear reader,
> > I have a question about combining methods and basis sets:
> >
> > does it make sense to run a calculation usin eg b3lyp using a cc-pVQZ
> > basis set. I am interested in opt, freq (raman) and nbo results.
> >
> >  Harald Born
> >
> >  Please send answers to
> >
> >  [email-masked]
> >
> 
> 
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> 
> 
> 
> 

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| Frank Jensen, Department of Chemistry          |
| SDU Odense University, DK-5230 Odense, Denmark |
| FAX +45 66 15 87 80 , Voice +45 65 50 25 07    |
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