Subject: DFT and H2O Dimer

From: "Dr. Heinz Schiffer" <[email-masked]>
Date: Tue, 07 Oct 1997 09:03:56 +0200

Hu Hao wrote:
> 
> Dear CCLers,
> 
> Weeks ago I've posted a mail about the DeFT optimization of water
> dimer. Thanks for replies from Dr. Frederic A. Van-Catledge and
> Dr. Rudolf Herrmann.
> 
> I have solved the problem now. There are two main reasons responsible
> to the results.
> 
> The first is the basis sets used. The problem could be partly solved
> by using larger basis sets, e.g., using DZVP4 or TZVP basis as AO
> basis, and A3 or A4 basis as auxiliary basis.
> 
> The second is as what Dr. Rudolf Herrmann said : "The functionals
> currently available for DF calculations are not meant to treat
> long range interactions such as Van der Waals forces, which should
> be highly important for your water dimer. DFT will be well suited
> for, lets say, an ice crystal, but not for this case of a water
> dimer." It seems that the system of water dimer needs careful
> consideration when studied.
> 
> Sincerely
> 
> Hao Hu
Hi CCL'ers,
I am not d'accord with Rudolf Herrmann's statement, that DFT is
not suitable for the water dimer, because "DFT methods are not meant
to treat long range interactions such as van der Waals forces". The
electrostatic part of the intermolecular interaction ist really the
most long range part and is treated the same way in DFT as in
conventional ab initio methods. What is really missing is Dispersion,
which is important for the interaction of non-polar molecules like
hydrocarbons and esp. for aromatic systems. Therefore the interactions
in the water dimer, which are dominated by the electrostatic and
polarization part of the interaction (i.e. H-bonds), are very well
described by DFT methods, like BP, BLYP, B3LYP, etc. (not LDA !, but
LDA is not DFT). On the other hand, the benzene dimer ( and of
course all noble gas dimers ) is only poorly described by DFT methods.
Best (!!!!) references :

	Michiel Sprik, Huerg Hutter, Michele Parrinello
	Ab initio molecular dynamics simulation of liquid water:
	Comparison of three gradient-corrected density functionals
	J. Chem. Phys. 105(3) (1996) 1142-1152
        ( It is not only about liquid water, nearly half of
	  the paper is about the water dimer )

	Evert Jan Meijer and Michiel Sprik
	A density-functional study of the intermolecular interactions
	of benzene
	J. Chem. Phys. 105(19) (1996) 8684-8689

Ciao,
Heinz	 
-- 
Dr. Heinz Schiffer                  Phone ++49-69-305-2330                      
Hoechst Research & Technology       Fax   ++49-69-305-81162                     
Scientific Computing, G864          Email [email-masked]         
65926 Frankfurt am Main                   [email-masked]

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From: Keith Refson <[email-masked]>
Date: Tue, 07 Oct 1997 10:57:28 +0100 (BST)

Dr. Heinz Schiffer writes:
 > Therefore the interactions
 > LDA is not DFT). 

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. 

For the water dimer, and "nonbonded" fragments in general, the various
GGAs are much closer to experiment than the LDA, and I agree with
Heinz Schiffer that they are pretty reasonable.

 >On the other hand, the benzene dimer ( and of
 
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.

Keith Refson
-- 
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From: "Nathaniel (noj) Malcolm" <[email-masked]>
Date: Tue, 7 Oct 1997 12:05:20 +0100

>course all noble gas dimers ) is only poorly described by DFT methods.

just a brief comment - on this comment to an earlier statement - 
i guess that one would
generally be keen to use DFT (rather than HF say) due to its inclusion
of correlation effects. As has also been discussed on the CCL recently the
'dispersion' effects are mainly driven by correlation effects 
therefore it seems to me that to say that DFT methods are 'good' for
the water dimer
problem because it gets most of the effects correct (except maybe
dispersion, which should not be the main force)
is placing too much faith in DFT - as it should really be able to deal
with dispersion as well (if it got the correlation right).

I guess my point is why not just use HF rather than DFT if its 
inclusion of correlation is not good enough to give a reasonable
physical description of dispersion forces?

noj

p.s. i'm sure that DFT does in practice offer improvement over HF its just that
i don't feel that Dr. Schiffer's argument explained why one should
consider it a 'good' method for these interactions.

(and i'm sure you could find the odd physicist out there who would be
able to argue that LDA IS DFT - maybe just not 'good' DFT for chemists)

-- 
Dr. Noj Malcolm			
Theory Group,			
Dept. of Chemistry,		
University of Manchester,	
Oxford Road,			
Manchester.			
M13 9PL			

e-mail [email-masked]