[Pw_forum] graphite cell optimization failed

[Nicola Marzari]

Dear Eric, All,


indeed - a good example are the phonons: in order to get
the right phonon dispersions in the Gamma-A direction
(perpendicular to the planes, i.e. where vdW should matter)
you need to have the right structure (i.e. the correct lattice
spacing, that in GGA would require vdW), but if you impose the
experimental lattice spacing you get the phonons correctly
(and thus some of the tricky elastic constants) even *without*
having a vdW term in the functional (btw, we discuss
quantum-espresso graphite in a lot of detail in
a 2005 PRB, with Nicolas Mounet - you could use those results
as a sanity check for your calculations).

				nicola


On 3/23/11 2:33 PM, Derek Stewart wrote:
> Hi Eric,
>
> I just wanted to follow up on Cyrille's comment. Density functional
> calculations for graphite based on the standard GGA exchange-correlation
> actually predict that the graphene layers don't bind together at all. So
> it is not surprising to see that you are getting very large unit cell
> values in the c direction. LDA exchange-correlation does a better job at
> getting the correct distance between graphene layers, HOWEVER, this is
> due to a lucky cancellation of errors. A quick test would be to rerun
> your structural relaxation with LDA to see if you get a more reasonable
> spacing between graphene layers.
>
> The root cause for your problems, as others on this forum, have
> mentioned is due to the fact that van der Waals interactions are not
> included in standard density functional theory (although QE now has some
> options available). In the end, the important question to ask is how
> much you expect van der Waals interactions to affect the property of
> graphite that you are interested. If it will make a minor contribution,
> then you may be able to just use the experimental distance between
> graphene layers in graphite or else use the LDA result.
>
> See the following article for a nice discussion on the LDA/GGA issue:
>
> "Hard numbers on soft matter", H. Rydberg, N. Jacobson, P. Hyldgaard, S.
> I. Simak, B. I. Lundqvist, and D. C. Langreth, Surface Science, 532-535,
> 606 (2003)
> http://dx.doi.org/10.1016/S0039-6028(03)00109-2
>
> If you don't have access to the journal, I think there may be local
> copies of the article on one of the author's website if you google based
> on the title.
>
> Best regards,
>
> Derek
-- 
----------------------------------------------------------------------
Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM