[DFTB-Plus-User] DFTB+NEGF: Dangling bonds in a graphene nanoribbon

Gabriele Penazzi gabriele.penazzi at bccms.uni-bremen.de
Sat May 10 17:31:27 CEST 2014


On 05/10/2014 10:25 AM, anz118466 at cse.iitd.ac.in wrote:
> Hi all,
>
> I have tried performing transport calculations of a graphene nanoribbon
> using the DFTB+NEGF package. As testcases, I took two scenarios: one, a
> GNR with the edge dangling bonds left intact and second, a GNR with H
> atoms attached to the edges. In both
> http://www.dftb-plus.info/pipermail/dftb-plus-user/2012/000816.htmlthe
> cases, I took an equal number of C atoms.
>
> As per the thread given on
> http://www.dftb-plus.info/pipermail/dftb-plus-user/2012/000816.html, DFTB
> does not specially treat dangling bonds. Yet, in my two testcases, SCC
> convergence is achieved in approx. 20 cycles in the second case (with H
> atoms) , and is not achieved at all in the first case (without H atoms)
> even after 500 cycles. Is this expected? Do the dangling bonds make this
> much of a difference?

Hi Anusha,

I didn't try to run nanoribbons with dangling bonds, but something 
similar and my experience is that it can make quite a difference.
If your system is metallic, add a small finite temperature (some 
Kelvin), it should help. Sometimes it is also a good idea to play around 
with the mixer parameters and use a lower mixing constant.

Normally, if it doesn't converge in a reasonable number of steps (<100), 
it will hardly converge at all.
You can also verify if you have the same problem in a periodic setup 
without open boundary conditions.
Open boundary conditions may be harder to converge.

Regards,
Gabriele


>
> Thanks and Regards,
> Anusha Iyer
>
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-- 
Dr. Gabriele Penazzi
BCCMS - University of Bremen

http://www.bccms.uni-bremen.de/
http://sites.google.com/site/gabrielepenazzi/

phone: +49 (0) 421 218 62337
fax: +49 (0) 421 218 62770



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