[Pw_forum] graphite cell optimization failed

[Mehmet Topsakal]

Dear all,

Once I had prepared a presentation in one of our group meetings which might
be helpful in this discussion :

http://www.unam.bilkent.edu.tr/mt2/random/vdW/

<http://www.unam.bilkent.edu.tr/mt2/random/vdW/>

On Wed, Mar 23, 2011 at 4:43 PM, Nicola Marzari <
nicola.marzari at materials.ox.ac.uk> wrote:

>
>
>
> 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
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-- 

Mehmet Topsakal  (Ph.D. Student)
UNAM-Institute of Materials Science and Nanotechnology.
Bilkent University. 06800 Bilkent, Ankara/Turkey
Tel: 0090 312 290 3527 ; Fax: 0090 312 266 4365
http://www.researcherid.com/rid/A-5015-2010
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