[Pw_forum] Multiple issues with graphene supercell calculations

[Dhruv Singh]

Thanks very much for the help regarding this topic.... I guess the
hexagonal supercell is a better choice then and I will need  to use a
finer k point grid with tighter scf convergence criterion for better
accuracy in forces. As an advice though, would you suggest that for
subsequent force constant calculations - forces of the order of 1.0e-5
would not lead to unstable modes ? (I am using a supercell to
calculate force constants rather than the ph.x codes because I later
need to calculate third derivatives)

As regards the 72 atom supercell scf calculations not converging, the
corresponding input file is attached here. I increased the scf
convergence threshold to 1.0e-3 in order to terminate the computation
earlier (to check the results). I am attaching two output files-one
with threshold 1.0e-8 and the other with 1.0e-3. Both of them use a 72
atom supercell and a 2x2x1 kpoint grid. Please let me know if you
think tweaking a few parameters could help convergence.

Thanks very much

Dhruv

On Mon, Nov 22, 2010 at 8:26 AM, Paolo Giannozzi <giannozz at democritos.it> wrote:
>
> On Nov 22, 2010, at 14:23 , Gabriele Sclauzero wrote:
>
>> Hence, if you want more precision you probably need to use
>> better k-point sampling and C-C distances.
>
> and better (i.e. smaller) self-consistenty convergence thresholds.
> The error on the energy is a quadratic function of the error on the
> charge density due to imperfect self-consistency; the error on forces
> is instead linear.
>
> P.
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics, Univ. Udine
> via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>



-- 
Dhruv
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gra.scf.in
Type: application/octet-stream
Size: 4821 bytes
Desc: not available
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20101122/c154e516/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gra2by2_thr1.0e-3.scf.out
Type: application/octet-stream
Size: 32858 bytes
Desc: not available
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20101122/c154e516/attachment-0001.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: gra2by2_thr1e-8.scf.out
Type: application/octet-stream
Size: 28264 bytes
Desc: not available
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20101122/c154e516/attachment-0002.obj>