# Re: [Wien] unphysical growth of mag. moment with supercell size

Hi,

it could be that the program cuts your rkmax for larger systems.
grep :RKM to check what was used.

regards

Robert

On 02 September 2016 PM 3:51:34 Maciej Zwierzycki wrote:
> Dear WIEN2k Users,
>
> I am trying to calculate the electronic structure (DOS, LDOS) of
> magnetic impurity/add on atom (Co) on the surface of silicene
> (i.e. Si graphene). While checking the convergence vs. the supercell
> size I've noticed the following artifact: the size of the magnetic
> moment on Co stays roughly constant (about 0.8 \mu_B) up until certain size
> of the supercell (say 9x9) and then starts growing, in some cases up to the
> atomic values (~3 \mu_B). I don't see how this could be physical, as the
> Co-silicene distance and the local geometry around impurity stays the same,
> however I was not able to find any obvious errors in my
> calculations.
>
> The calculations for bulk silicene work well.
>
> I'm running 14.2 version, the vertical separation between silicene
> sheets is 14 Angst, the lattice constant is a=3.86 Angst. I've used
> rKmax between 3 and 7. The k-mesh was generated using (3,3,1)
> divisions but I've also tested with denser 2D meshes and the
> automatically generated meshes. The symmetry, determined during the
> initialization phase was the same in all the cases, e.g.:
>
> H   LATTICE,NONEQUIV.ATOMS: 61 156 P3m1
>
> I've started these calculations by extracting the geometry of local Co
> neighborhood from the relaxed calculations with 4x4 supercell and
> embedding it into bulk silicene when running larger supercells. The
> calculations were then performed with thus fixed geometry. However the
> same trends can be observed with unrelaxed geometry. In the latter
> case I fashioned the supercell  out of bulk unit cell and then
> attached Co atom at arbitrary height.
>
> I'd be happy to provide further informations (structure files) and grateful
> for any hints you may give me.
>
> Kind Regards
> Maciej Zwierzycki
>
> PS. At the moment I'm trying to establish to what extent the results
> are sensitive to separation between silicene sheets.
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