[Pw_forum] Multiple issues with graphene supercell calculations

[Dhruv Singh]

And as expected, the termination threshold of 1.0e-3 gives a rather
lousy answer for forces and energy/atom of -11.39486969 Ry
significantly different from the correct value of ~ -11.395192 Ry. I
believe a finer kpoint grid should resolve the convergence issue to a
certain extent (correct me if I am wrong) - but I am trying to
understand if  a well converged asnwer can be obtained even on a 2x2x1
kpoint grid.

Thanks,
Dhruv

On Mon, Nov 22, 2010 at 1:18 PM, Dhruv Singh <dhruv.singh at gmail.com> wrote:
> 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
>>
>>
>>
>>
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>
>
>
> --
> Dhruv
>



-- 
Dhruv