Subject: [Fwd: Re: CCL:DFT and H2O Dimer]

From: "Dr. Heinz Schiffer" <[email-masked]>
Date: Wed, 08 Oct 1997 15:33:10 +0200

Hi Keith,
 
> Perhaps I am being picky here but BP, BLYP, B3LYP etc are no more or
> no less DFT than LDA is. *All* are approximations to the "true" DFT
> exchange-correlation functional, which no-one knows -- it's just that
> some are better approximations than others.

O.k., that is exactly, what I meant. LDA and DFT are not synonyms.
 
> Interesting.  But the LDA makes a very good job of graphite, including
> the interlayer bonding, which goes to show that not every force that
> appears weak is really Van der Waals.

What do you mean by van der Waals ? I propose, one should speak
about bonding (covalent) and non-bonding forces. The non-bonding
forces can be divided into electrostatic, or better call it Coulomb, 
forces, into (Pauli) repulsion, into dispersion, into polarization, etc.
What is weak and what is strong ? It depends: at very short distances
the (Pauli) repulsion is very strong, at very long distances the
covalent bond forces are very weak, etc.
Why LDA is good for graphit, I do not know. One knows, that LDA
leads always to a strong overbinding for noble gas dimers. May be
the good performance of LDA for graphit is the result of error
compensations ( no dispersion, underestimation of the repulsive cores,
underestimation of the quadrupole moment ).

Ciao,
Heinz

-- 
Dr. Heinz Schiffer                  Phone ++49-69-305-2330                      
Hoechst Research & Technology       Fax   ++49-69-305-81162                     
Scientific Computing, G864          Email [email-masked]         
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