[Pw_forum] van der waals force

[Axel Kohlmeyer]

On Sat, 2009-05-16 at 15:57 +0330, Mehrnaz Anvari wrote:
> Dear paulatto
> Thanks a lot because of your attention. I want to put graphene on
> substrate so I think that the van der waals force becomes important
> for this case (isn't correct?). By the way I have read the paper of
> M.Lazzeri & et al(Phys. Rev. B 78, 081406(R) (2008)) & I know the
> problem of LDA & GGA approximations(for graphene & graphite), so if
> there is any way to correct them please aware me.

mehrnaz,

several approaches have been tried to improve the representation of
dispersion interactions in DFT. they range from hybrid-DFT, over
additional correction terms derived from the self-consistent density
to empirically modified pseudopotentials and adding classical force
field terms. they vary in computational effort and packages where
they have been implemented in. which of the methods are "best" depends
a lot on what you would like to compute and how much computational
power you have available. however, one should note that the methods
with more computational effort do not always improve the results over
"cheap" ones in certain cases. one has to understand how a given 
correction tries to address the problem to be able to make a choice.  

in short: there is a lot of literature out there. dig in!

cheers,
    axel.

> Well, I have to explain more about my first question. please assume
> that we have two layers of graphene. First the distance between them
> is 3 a.u & then it has been changed to 10 a.u. What will happen for
> hartree(Uee) term in calculations, Do you expect that it will increase
> or decrease?
> Best Regards
> Mehrnaz Anvari
> Iran University Of Science & Technology 
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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
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