[Pw_forum] Functionals, pseudopotentials, etc. for H2O ice

[Axel Kohlmeyer]

On Sun, 10 Jul 2005, Konstantin Kudin wrote:

kostya,

thank you for proving the point that 
this _is_ a controversial topic. ;-)

KK>  Well, it is not that useful to lump all types of weak interactions
KK> into dispersion that DFT supposedly does not describe. As a matter of
KK> fact, hydrogen bonds do have a rather strong Coulomb and density
KK> overlap component, so plain DFT description of that can be quite
KK> reasonable. Such bonds are quite different from those in something like
KK> He dimer, where the lowest level of physically proper description is
KK> some kind of perturbation theory like MP2. Hybrid functionals do work
KK> somewhat better, however, the expense is much greater as well.

it was not was not my intention to make plain DFT look bad.
it really depends on what you want to investigate, so you
have to test what works well. the representation
of hydrogen bonds can vary a lot between functionals.
one example of comparing the performance of functionals
and methods, which also includes a water dimer, is in 
X. Wu et al., JCP 115(2001), p8748

KK>  I think there was a recent paper assessing a new meta-GGA functional
KK> TPSS for hydrogen bonds, and it probably also contains data for other
KK> functionals as well (Staroverov, Scuseria, JCP, 2004 [or 2003]). If I
KK> recall correctly they concluded that TPSS could be as good as hybrids
KK> for hydrogen bonds while being cheaper computationally.

many seem to be satisfied with BLYP, which gets a positive rating 
for the water dimer in the paper above, another example would be 
the HCTH functional which - to the best of my knowledge - is still
continously refined, another alternative, as one example for a method
where the potential is modified, would be the method presented
in PRL 93 (2004), 153004, and this list is definitely not complete.

my main point remains, that there still is not _the_ 
method/functional/potential that works well for water
under _all_ circumstances. so that one has to be careful
in validating whatever approach one uses. not to mention 
the issues addressed by nicola (it is particularly 
embarrassing that i forgot to think about proton tunneling,
considering who has signed my current contract. ;-)) )

best regards,
	axel.


KK> 
KK>  Kostya
KK> 
KK> 
KK> 		
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-- 

=======================================================================
Dr. Axel Kohlmeyer   e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.