Subject: Re: CCL:dft question on sugar

From: Per-Ola Norrby <pon (a) kemi.dtu.dk>
Date: Thu, 3 Feb 2005 08:32:13 +0100

>My question is: which  is or which are the best functionals for dft that
>describe correctly hydrogen bond and weak interaction like van der walls
>etc
>in sugar moieties?

	vdW forces do NOT come out of current HF or DFT approaches. 
It's not only that they are badly described, they are simply not 
there at all.  Some functionals can, due to error compensations, give 
a minimum at approximately the correct distance, but this is 
definitely "the right result for the wrong reason", and you cannot 
expect any kind of predictivity.  One example is the BSSE effect you 
find in HF calculations with small or unbalanced basis sets, it looks 
very much like vdW.

	I know that development of DFT functionals that include vdW, 
and in particular London dispersion, are appearing, some were 
reported the last couple of years, but I don't think any are included 
in current available software.

	My recommendation for a reasonably reliable vdW treatment is 
to use empirical force fields.  I've seen approaches where an MM term 
is added to a standard DFT, a sort of QM/MM approach.  The method I'd 
use myself would be to parameterize a force field to reproduce the 
bonding interactions in your substrate as given by DFT, then trust 
the force field for the vdW.  I've included references to a couple of 
reviews about these methods below, and a user-UNfriendly program for 
doing this can be picked up from my web site.  Beware though, I don't 
know of any force field method that gives the accuracy I demand for 
hydrogen bonded systems.  Maybe one of the force fields from Kjeld 
Rasmussen, but I don't know for sure.

# "Deriving Force Field Parameters for Coordination Complexes"
Per-Ola Norrby and Peter Brandt, Coord. Chem. Rev. 2001, 212, 79-109

# "Recipe for an organometallic force field"
Per-Ola Norrby, in Computational Organometallic Chemistry, Ed: T. 
Cundari; Marcel Dekker: New York, 2001, 7-37

	Regards,

	Per-Ola
-- 
Per-Ola Norrby, Assoc. Professor, http://organisk.kemi.dtu.dk/PON/
Technical University of Denmark, Department of Chemistry
Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark
Email: pon (a) kemi.dtu.dk  tel +45-45252123,  fax +45-45933968