[Pw_forum] Multiple issues with graphene supercell calculations

[Gabriele Sclauzero]

Il giorno 22/nov/2010, alle ore 19.23, Dhruv Singh ha scritto:

> 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

I wouldn't say "lousy"... you got what you asked for, that is to a say an accuracy of 1e-3 Ry on the total energy.

> 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.

if you use a n x n supercell, you should get the same accuracy of the primitive unit cell by reducing the k-point mesh accordingly (e.g.  k_x/n k_y/n)

GS

> 
> 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
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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