[Pw_forum] Multiple issues with graphene supercell calculations

[Gabriele Sclauzero]

Il giorno 22/nov/2010, alle ore 19.18, Dhruv Singh ha scritto:

> 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

If there's no specific reason for using a lower symmetry cell (as the tetragonal one you've been using previously), I think so.

> 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

That's a very tight threshold! i.e. 2 orders of magnitudes smaller than the default one...

> 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.

Graphene has zero bandgap, hence you might need to include a broadening of the electronic occupations (i.e. you need to specify also occupations and smearing keywords, degauss is not sufficient).

HTH

GS

> 
> 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
> <gra.scf.in><gra2by2_thr1.0e-3.scf.out><gra2by2_thr1e-8.scf.out>_______________________________________________
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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