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 > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > -- 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110323/2226ef1f/attachment-0001.htm
