[Pw_forum] Forces are not zero (for atoms in non-primitive cell)

[Lorenzo Paulatto]

On 11/20/2013 05:54 PM, Duy Le wrote:
> Dear all,
>
> I wonder if anyone knows how to vanish the forces acting on atoms in a
> non-primitive cell. I saw that if I have 1x1x5 cell of Cu(111) (or any
> other systems), the x-y components of forces are zero but when I make
> 2x2x5 or other supercell, these forces becomes non-zero. Of course,
> these values are quite small. I used lot of significant digits for
> coordinates and cell parameters but they do not help.

To get exactly the same result for the bigger cell you must assure that 
the FFT grid scales proportionally and the K points grid scales inversely.

I.e. lest say that the unit cell has nr1=10, nr2=10,nr3=10 (either 
specified by hand or automatically selected, it does not matter) and a 
10x10x10 grid of k-points. If you build a 2x2x2 supercell you will have 
to check that nr1=20, nr2=20, nr3=20 and in case enforce it manually. 
You will also have to use 5x5x5 k-points, keeping in mind that if the 
grid was shifted it may be impossible to have exact equivalence.

good work


-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
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