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

[Duy Le]

Thank Lorenzo,

I did assure those settings. I can get identical energy. However, the
x-y forces are not going to zero.

Duy
----------------------------------------------------
Duy Le
Postdoctoral Associate
Department of Physics
University of Central Florida.
Website: http://www.physics.ucf.edu/~dle


On Wed, Nov 20, 2013 at 12:16 PM, Lorenzo Paulatto
<lorenzo.paulatto at impmc.upmc.fr> wrote:
> 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
> +33 (0)1 44 275 084 / skype: paulatz
> http://www-int.impmc.upmc.fr/~paulatto/
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