<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body text="#000000" bgcolor="#ffffff">
Hi Eric,<br>
<br>
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. <br>
<br>
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.<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. I. Simak, B. I. Lundqvist, and D. C. Langreth, Surface
Science, 532-535, 606 (2003) <br>
<a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1016/S0039-6028(03)00109-2">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
copies of the article on one of the author's website if you google
based on the title.<br>
<br>
Best regards,<br>
<br>
Derek<br>
<br>
<pre class="moz-signature" cols="72">################################
Derek Stewart, Ph. D.
Senior Research Associate
** New Webpage **
<a class="moz-txt-link-freetext" href="http://sites.google.com/site/dft4nano/">http://sites.google.com/site/dft4nano/</a>
250 Duffield Hall
Cornell Nanoscale Facility (CNF)
Ithaca, NY 14853
stewart (at) cnf.cornell.edu
(607) 255-2856
</pre>
<br>
On 3/23/2011 12:33 PM, Eric Germaneau wrote:
<blockquote cite="mid:4D8A20BE.9020504@gucas.ac.cn" type="cite">
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
Dear all,<br>
<br>
I'm trying to optimize unit cell parameter of graphite.<br>
The problem is the c axis gets too long.<br>
Starting from published data my initial parameters are a=4.59203
Bohr and c=12.513 Bohr.<br>
The final values are 4.664 and 15.829 respectively.<br>
I need someone to explain to me what I did wrong, I've attached my
input file.<br>
That's a basic calculation so it's very frustrating to not make it
done.<br>
I thank you in advance,<br>
<br>
Eric.<br>
<br>
<div class="moz-signature">-- <br>
<center> <font color="#946e8c"> <em> Be the change you wish to
see in the world<br>
</em> <small> <font color="#946e8c"> — Mahatma Gandhi —<br>
</font></small> <br>
<a moz-do-not-send="true"
href="mailto:germaneau@gucas.ac.cn">Dr. Éric Germaneau</a><br>
<br>
College of Physical Sciences<br>
Graduate University of Chinese Academy of Sciences<br>
Yuquan Road 19A<br>
Beijing 100049<br>
China<br>
<br>
<small> <em> <font style="color: rgb(73, 136, 72);"
face="Times"> Please consider the environment before
printing this email.<br>
Considérez svp l'environnement avant d'imprimer cet
email. </font> </em> </small> </font></center>
</div>
<pre wrap="">
<fieldset class="mimeAttachmentHeader"></fieldset>
_______________________________________________
Pw_forum mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a>
<a class="moz-txt-link-freetext" href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</a>
</pre>
</blockquote>
</body>
</html>