[Pw_forum] graphite cell optimization failed

[Derek Stewart]

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. Éric Germaneau <mailto:germaneau at gucas.ac.cn>
>
> College of Physical Sciences
> Graduate University of Chinese Academy of Sciences
> Yuquan Road 19A
> Beijing 100049
> China
>
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>
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