Subject: Re: CCL:DFT is over !/? (expert opinion needed)

From: Stefan Grimme <[email-masked]>
Date: Thu, 22 Mar 2001 13:16:51 +0100

> Dear CCLers,
>  I just became aware of the recent paper by Clementi et al. (see
> the relevant abstracts below).
>  It seems to me that the quest for correlation functional is over:
> Clementi just did it (without any density, but with 16
> empirical parameters).
>  He...He, water trimer, and atomization energies to +/- 0.05 kcal/mol
> sound pretty convincing.
>  What do you think?

Dear Artem,
I recently had the opportunity to talk with Clementi about his approach
and I also read a recent preprint (I think the one you mentioned). I'm not
really convinced that the Coulomb-hole approach with a lot of parameters 
represents a reasonable quantum chemical approch. The main points of my
criticism are as follows:

1. The dispersive interactions are modelled by an empirical formula
which means that the approach is not more than a combination of
force field technology and QM in that respect, i.e. its a new version of QM/MM.

2. The Coulomb-hole (CH) improves on the HF atomization energies but does
NOT improve the relative energies which we are most interested in (Clementi
strongly emphasizes on atomisation energies but forgot to look to the relative
energies!). Simple organic reaction energies are even WORSE with the CH-HF
method compared to HF. Even the simplest DFT's perform better in that respect.

3. An important part of the new CH-HF formalism is based on Mulliken
charges and bond orders. We all know the problems associated with this
approach (strong basis set dependence) and I therefore expect that
each basis set would need its own parametrization (this problem has not
been discussed in the paper I know).

4. Some time ago we tested a simpler (meaning a smaller number of parameters)
version of CH-HF for He and H-. The result was that the radial densities were
even worse than those from HF when compared with almost exact densities. 
My impression was that the basic correlation effects are NOT included
accurately, even not in a qualitative manner. Again, DFT improves upon HF, at
least qualitatively. Thus, I don't expect any improvement for properties with
the CH-HF method (I think this question is important and Clementi has to
investiagte this. We know that DFT does not only improve energies but also
properties like dipole moments etc.; the reason is simply that the DFT
densities are on the whole better than HF densities).

In summary I think the approach is worth while testing it but I don't think
that "the quest for correlation functional is over".
Stefan

_________________________________________________________
Prof. Dr. Stefan Grimme
Organisch-Chemisches Institut (Abt. Theoretische Chemie)
Westfaelische Wilhelms-Universitaet, Corrensstrasse 40
D-48149 Muenster, Tel (+49)-251-83 36512/33241/36515(Fax)
Email:[email-masked]
http://www.uni-muenster.de/Chemie/OC/research/grimme/
_________________________________________________________
Positions available for carrying out doctoral or 
postdoctoral studies in chemistry in Münster: 
http://www.uni-muenster.de/Chemie/OC/positions/
_________________________________________________________