Grimme correction and supercell

[Axel]

On Mar 16, 12:54 pm, Andrzej <kwa... at o2.pl> wrote:
> Dear users,
>
> I performed some calculation for graphite, just for testing some
> features.
> Here are some results obtained from single point calculations made for
> experimenrtal crystal structure under periodic conditions.
> Calculations were performed for single unit cell (111), and extended
> unit cells: 222, 332 and 442.
>
> Here are values of dispersion (Grimme) energy per C atom.
>
> 111: -0.004719
> 222: -0.0041627
> 332: -0.00405803
> 442: -0.00405803
>
> As the calculations were performed under periodic conditions I would
> expected that all these values should be the same. Grimme correction
> is purely classical expression independent of electron density,
> cutoff,  lack of k-point sampling and so on.

but it is not independent of minimum image convention and
the classical cutoff. i haven't checked the cp2k implementation,
but the equivalent code in cpmd would provide an option to
compute the real space interactions across multiple copies of
the principle unit cell, so that for small unit cells, you
would still be able to compute those interactions up to a
significant distance beyond the restrictions of using only
the closest image.

cheers,
   axel.

>
> I will be grateful for any comment.
>
> Andrzej