Subject: Re: Problems with DFT for reaction barriers

From: [email-masked]
Date: Wed, 20 Mar 1996 16:08:57 -0500 (EST)

Dear Dr. Creve,

>From my own work and that of others, is has emerged that DFT has problems 
>in investigating transition structures for H-radical reactions (additions 
>and eliminations) with a low energy barrier. Often, the lack of exact 
>Hartree-Fock exchange in some functionals is blamed for this.

We published a study on this type of problem after finding that
regular Kohn-Sham DFT is generally a complete failure for radical H
abstractions.  We took a step back and examined the simplest such
reaction, H + H2 -> H2 + H, and found that much of the problem can be
traced to the spurious self-interaction of the electrons in
approximate density functionals.  The effect can be dramatic: for the
LSDA, regular KS theory predicts H3 to be *stable* with respect to
H + H2 (!) but after a simple approximate self-interaction correction
(SIC) to the potential surface the transition structure was restored.
The reference is

B.G. Johnson, C.A. Gonzalez, P.M.W. Gill and J.A. Pople,
Chem. Phys. Lett. 221, 100 (1994).

I don't really agree with the lack of HF exchange as the explanation
for the poor performance of DFT in the case of reaction barriers.
"Hybrid" functionals like B3LYP have been shown to give good results
on some reaction barriers in practice, but it is worrisome that the
methods being mixed in these hybrids, i.e. Hartree-Fock and pure DFT,
give reaction barriers which are significantly higher than experiment
in one case (HF) and significantly lower than experiment in the other
(DFT).  This leads one to wonder whether the good hybrid results
simply come from fortuitous cancellation of errors -- the effect of
the spurious residual self-energy in DFT roughly cancelling that of
the neglect of electron correlation in Hartree-Fock.

Having said this, SIC is certainly more difficult and expensive to do
rigorously in practice than to use a hybrid HF-DFT functional; I do
think, though, that one should be cautious of the results when using
hybrid methods for this type of problem, for the reasons mentioned
above.

Regards,
Benny Johnson
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