[DFTB-Plus-User] graphene melting

[Jan M. Knaup]

Dear Sandeep,

contrary to Bálint's opinion, I believe that your problem is caused by the
thermostat, so a stronger coupling would only make things worse. It is
well-known that thermostats influence dynamic properties at equilibrium and
that non-equilibrium dynamics cannot be handled by applying a thermostat to
the system. The melting of a material is arguably not an equilibrium
property, so you have to be very careful.
The easiest way to get around your problem is to use a fairly strongly
coupled thermostat to equilibrate your model at the target temperature,
then restart the simulation in NVE, using the density matrix and velocities
from the equilibration run. Depending on your system size, you will end up
within a reasonable range of your desired target temperature. With this
procedure, the melting will of course lower your temperature, as the
melting heat will not be replaced by the thermostat.
An alternative that offers more control would be to use an approach similar
to the one described by Page et.al. in J. Chem. Theory Comput., 2012, 8 (11),
pp 4019–4028 (http://pubs.acs.org/doi/abs/10.1021/ct3004639) where you add
an explicit heat bath in the form of a thermostated inert gas, but I
believe that is not possible with the stable version of DFTB+.
At least you can try using a thermostat that influences the dynamics of
your system less harshly, e.g. Nosé-Hoover. However, I think that
simulating the graphene in an NVE ensemble is much better.

As a side note, I think your time step is very small, have you tried using
something like 0.5 fs?

Best regards,
Jan

Jan M. Knaup                      | Fon +49-(0)421-218-62351
Dipl. Phys. Dr. rer. nat.         | Fax +49-(0)421-218-62770
Universität Bremen - BCCMS        |
Am Fallturm 1                     | Jan.Knaup at bccms.uni-bremen.de
28359 Bremen                      | JanKnaup at gmail.com
Germany                           | www.bccms.uni-bremen.de


2013/1/27 sandeep singh <san2007iit at gmail.com>

> Hi DFTB+ users,
>
> I am trying to calculate melting behaviour of graphene. I used time
> step=0.1fs and run the MD code using Anderson thermostat  upto temperature
> 6000K. I didnt see any defect formation (5-7 defect or Stones-Walls defect)
> during melting. Is DFTB+ parameters for  carbon  bond breaking are included
> in file? or anybody can suggest good thermostat or any mechanism to get
> correct results.
>  Thanks in advance
> --
> Regards
>
> Sandeep Kumar Singh
> PhD Student
> Condensed Matter Theory Group
> CGB building U 2nd floor
> University of Antwerpen
> Groenenborgerlaan 171
> B-2020 Antwerpen
> Belgium
>
> Email: SandeepKumar.Singh at ua.ac.be
> san2007iit at gmail.com
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