[Pw_forum] van der waals force

[Lorenzo Paulatto]

In data 25 maggio 2009 alle ore 12:24:14, Mehrnaz Anvari  
<anvari_meh at physics.iust.ac.ir> ha scritto:
> 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.

If I new an exchange-correlation functional guaranteed to reproduce  
experimental values I would be the younger professor of condensed matter  
physics... unluckily all available XC functionals involve some level of  
aproximation and can only reproduce experiments up to that level. There is  
a lot of literature on this topic; I suggest consulting the book by prof.  
Richard Martin.

> Now I think in VdW force you should have segnificient changes in your  
> core density, because of polarization.

I really don't know, VdW interaction is assumend to be relatively weak in  
the scale of electronic interaction. Hence they should not affect the  
ionic cores much. Again, an uncountable number of papers have been written  
on this topic, sometimes VdW forces can be neglected, sometimes they are  
the only term keeping the system together.

> It seems that situation could be same when we use full potential (Isn't  
> true?)

You can use PAW instead of ultrasoft pseudopotential to improve on the  
core aproximation. Keep in mind that pseudopotential and frozen-core are  
two different kinds of aproximation: the ultrasoft/norm-conserving methods  
use them both, while PAW only uses frozen-core.

In general, this has nothing to do with VdW forces. If your concern is the  
lack of VdW interactions, I don't see how improving the threatment of core  
orbitals can help (but I may be mistaken).

Furthermore, QE does not implement any empiric or equally cheap VdW  
correction (left alone doing phonon calculations with it), hence if you  
need it you have to look somewhere else, or implement it yourself.

regards

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
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