Subject: (Fwd) CCL:DFT and H2O Dimer

From: "Dr. Martin Schuetz" <[email-masked]>
Date: Wed, 8 Oct 1997 11:06:01 -0600

>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 :

DFT would lead primarily to an improvement over SCF due to introduction of
intramolecular correlation, which modifies the multipole moments of the
individual water monomers. These are "local" correlation effects and hence
treated well by DFT. However this portion of the interaction energy is
repulsive (0.3-0.4 kcal/mol according to SAPT) and therefore the
DFT interaction energy should be lower than the SCF interaction energy,
if there is no dispersion energy carried by the functionals.
In the water dimer, the dispersion energy still is of the order of 25-30 %
of the water dimer interaction energy, which although not dominant, certainly
is a non-negligible fraction of the interaction energy. If the present
functionals are not able to describe the less local dispersion forces
appropriately, one should be careful with H-bonded systems as well when using
DFT. In that sense I disagree with Dr. Schiffer, that the water dimer really is
well described with DTF with the present standard gradient corrected
functionals.
A good method to describe the water dimer and larger water clusters is
MP2, which is very close to CCSD(T) for these systems, since SD and disconneted
Q excitations cancel the connected triples to large extends.

---

From: "Dr. Heinz Schiffer" <[email-masked]>
Date: Wed, 08 Oct 1997 15:28:07 +0200

Hi Martin,

> well described with DTF with the present standard gradient corrected

You call that not well described ? :

------------------------------------------------
			DFT-BP		Exp.
------------------------------------------------
O-O distance		2.98 A		2.98 A
angle of the H-bond	174 deg		174 deg
Binding energy		-20 kJ/mol	-22 kJ/mol
Frequencies, A		3671		3622
	A		1605		1600
	A		3779		3714
	D		3501		3548
	D		1588		1618
	D		3755		3698
	Hb		666		520
	Hb		396		320
	O-O		205		243
------------------------------------------------

The data are from the paper of Sprik, Hutter, and Parrinello.
And DFT-BP is an order of magnitude faster than MP2. That is not
to say, that MP2 doesn't make a very good job.

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]