[Pw_forum] van der waals force

[Axel Kohlmeyer]

On Mon, 2009-05-25 at 13:54 +0330, Mehrnaz Anvari wrote:
> Thanks a lot because of your attention. I am trying to do phonon
> calculation in graphene so I think, having well approximation for
> pseudo potential & exchange-correlation one, will help you to achieve
> good frequencies which are compatible with experiment.
> Now I think in VdW force you should have segnificient changes in your
> core density, because of polarization. It seems that situation could

why so? why should the core electrons be more polarizable?
i'd say on the contrary. please note that dispersion interactions
are interactions between induced dipoles, thus quite weak compared
to point charge interactions. now, the closer you get to the core, 
the less the core charge is screened by the electrons and thus the
changes due to polarization would matter increasingly less the closer
you get to the core. this is why van der waals interactions matter 
the most for large and polarizable elements and diffuse electron
densities.

>  be same when we use full potential (Isn't true?) 

no. as nicola pointed out, it can matter how many electrons you put
into the valence depending on the element, but the (inner) core
electrons for sure do not matter. keep in mind that people successfully 
study much more radical polarizations than what dispersion does with
pseudopotentials. if you were right, all calculations with
pseudopotentials would be useless.

cheers,
   axel.

> Best Regards
> Mehrnaz Anvari
> Iran University Of Science & Technology 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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