Dear all,<div><br></div><div>Once I had prepared a presentation in one of our group meetings which might be helpful in this discussion :</div><div><br></div><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><a href="http://www.unam.bilkent.edu.tr/mt2/random/vdW/">http://www.unam.bilkent.edu.tr/mt2/random/vdW/</a></div>
<div><br></div><div><a href="http://www.unam.bilkent.edu.tr/mt2/random/vdW/"></a><br><br><div class="gmail_quote">On Wed, Mar 23, 2011 at 4:43 PM, Nicola Marzari <span dir="ltr"><<a href="mailto:nicola.marzari@materials.ox.ac.uk">nicola.marzari@materials.ox.ac.uk</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><br>
<br>
<br>
Dear Eric, All,<br>
<br>
<br>
indeed - a good example are the phonons: in order to get<br>
the right phonon dispersions in the Gamma-A direction<br>
(perpendicular to the planes, i.e. where vdW should matter)<br>
you need to have the right structure (i.e. the correct lattice<br>
spacing, that in GGA would require vdW), but if you impose the<br>
experimental lattice spacing you get the phonons correctly<br>
(and thus some of the tricky elastic constants) even *without*<br>
having a vdW term in the functional (btw, we discuss<br>
quantum-espresso graphite in a lot of detail in<br>
a 2005 PRB, with Nicolas Mounet - you could use those results<br>
as a sanity check for your calculations).<br>
<br>
nicola<br>
<div><div></div><div class="h5"><br>
<br>
On 3/23/11 2:33 PM, Derek Stewart wrote:<br>
> Hi Eric,<br>
><br>
> I just wanted to follow up on Cyrille's comment. Density functional<br>
> calculations for graphite based on the standard GGA exchange-correlation<br>
> actually predict that the graphene layers don't bind together at all. So<br>
> it is not surprising to see that you are getting very large unit cell<br>
> values in the c direction. LDA exchange-correlation does a better job at<br>
> getting the correct distance between graphene layers, HOWEVER, this is<br>
> due to a lucky cancellation of errors. A quick test would be to rerun<br>
> your structural relaxation with LDA to see if you get a more reasonable<br>
> spacing between graphene layers.<br>
><br>
> The root cause for your problems, as others on this forum, have<br>
> mentioned is due to the fact that van der Waals interactions are not<br>
> included in standard density functional theory (although QE now has some<br>
> options available). In the end, the important question to ask is how<br>
> much you expect van der Waals interactions to affect the property of<br>
> graphite that you are interested. If it will make a minor contribution,<br>
> then you may be able to just use the experimental distance between<br>
> graphene layers in graphite or else use the LDA result.<br>
><br>
> See the following article for a nice discussion on the LDA/GGA issue:<br>
><br>
> "Hard numbers on soft matter", H. Rydberg, N. Jacobson, P. Hyldgaard, S.<br>
> I. Simak, B. I. Lundqvist, and D. C. Langreth, Surface Science, 532-535,<br>
> 606 (2003)<br>
> <a href="http://dx.doi.org/10.1016/S0039-6028(03)00109-2" target="_blank">http://dx.doi.org/10.1016/S0039-6028(03)00109-2</a><br>
><br>
> If you don't have access to the journal, I think there may be local<br>
> copies of the article on one of the author's website if you google based<br>
> on the title.<br>
><br>
> Best regards,<br>
><br>
> Derek<br>
</div></div><font color="#888888">--<br>
----------------------------------------------------------------------<br>
Prof Nicola Marzari Department of Materials University of Oxford<br>
Chair of Materials Modelling Director, Materials Modelling Laboratory<br>
<a href="mailto:nicola.marzari@materials.ox.ac.uk">nicola.marzari@materials.ox.ac.uk</a> <a href="http://mml.materials.ox.ac.uk/NM" target="_blank">http://mml.materials.ox.ac.uk/NM</a><br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br><br>Mehmet Topsakal (Ph.D. Student)<br>UNAM-Institute of Materials Science and Nanotechnology.<br>Bilkent University. 06800 Bilkent, Ankara/Turkey<br>Tel: 0090 312 290 3527 ; Fax: 0090 312 266 4365<br>
<a href="http://www.researcherid.com/rid/A-5015-2010" target="_blank">http://www.researcherid.com/rid/A-5015-2010</a><br><br>
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