[Pw_forum] Multiple issues with graphene supercell calculations

[Gabriele Sclauzero]

On 11/22/2010 12:01 PM, Gabriele Sclauzero wrote:
> Dear Singh,
>
>    i) in the hexagonal cell the forces vanish by symmetry, that's why 
> they are perfectly zero. In the rectangular cell this is no more the 
> case. I think the problem arises because you did not specify the 
> celldm(1) parameter with enough significant digits, perhaps, hence the 
> structure does not correspond exactly to the optimal one.
>    Then keep in mind that the cell is rectangular, thus in principle 
> you should use a different number of k-points in the two directions.
>
>
Indeed I found that if you use a 15x9x1 k-point grid, which is roughly 
equivalent to the 16x16x1 in the hexagonal cell, the forces are

      Forces acting on atoms (Ry/au):

      atom   1 type  1   force =     0.00037456    0.00000000    0.00000000
      atom   2 type  1   force =    -0.00037456    0.00000000    0.00000000
      atom   3 type  1   force =     0.00037456    0.00000000    0.00000000
      atom   4 type  1   force =    -0.00037456    0.00000000    0.00000000

that is to say 10 times smaller than what you cited. Hence, if you want 
more precision you probably need to use better k-point sampling and C-C 
distances.

GS

-- 

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