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 ################################ Derek Stewart, Ph. D. Senior Research Associate ** New Webpage ** http://sites.google.com/site/dft4nano/ 250 Duffield Hall Cornell Nanoscale Facility (CNF) Ithaca, NY 14853 stewart (at) cnf.cornell.edu (607) 255-2856 On 3/23/2011 12:33 PM, Eric Germaneau wrote: > Dear all, > > I'm trying to optimize unit cell parameter of graphite. > The problem is the c axis gets too long. > Starting from published data my initial parameters are a=4.59203 Bohr > and c=12.513 Bohr. > The final values are 4.664 and 15.829 respectively. > I need someone to explain to me what I did wrong, I've attached my > input file. > That's a basic calculation so it's very frustrating to not make it done. > I thank you in advance, > > Eric. > > -- > /Be the change you wish to see in the world > / --- Mahatma Gandhi --- > > Dr. ?ric Germaneau <mailto:germaneau at gucas.ac.cn> > > College of Physical Sciences > Graduate University of Chinese Academy of Sciences > Yuquan Road 19A > Beijing 100049 > China > > /Please consider the environment before printing this email. > Consid?rez svp l'environnement avant d'imprimer cet email. / > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20110323/c8424af4/attachment.htm
