[Pw_forum] Convergence problem in Si slab structure

[reza vatan]

Dear Giovanni,

Thank you very much. It helps a lot.

Best,
Reza

On Wed, May 13, 2015 at 3:25 AM, Giovanni Cantele <
giovanni.cantele at spin.cnr.it> wrote:

>
> > On 13 May 2015, at 02:41, reza vatan <rezavatan64 at gmail.com> wrote:
> >
> > Dear QE users,
> >
> > I'm trying to optimize a Silicon slab structure with 13 atomic layers in
> each slab (attached file), but so far I have no success in optimizing
> because of convergence problem in SCF cycle. Does any one know who I can
> fix the problem.
> >
> > Many thanks,
> > Reza
> > <si-rel.in>_______________________________________________
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>
>
> As far as I understand you are dealing with the Si(100) surface, aren’t
> you?
>
> In the case, you might want to passivate the two sides of the slab with
> hydrogens. If not, dangling bonds that necessarily appear can be more
> difficult to converge. Also, consider that for that surface, surface Si
> atoms are known to form dimers, that is, to pair and form surface
> supercells (2x1, 2x2, 4x2, see the literature).
>
> If instead you want to converge exactly the system shown in your input
> file, you should better use occupations=’smearing’ (otherwise the degauss
> variable you next specify is useless!). Even though your surface is not
> metallic, that can help to converge when there are near gap electronic
> states, that “oscillate” in the scf convergence procedure.
>
> Moreover, the cutoff you use might be insufficient to get converged
> results.
>
> Finally, to save computing time, the 4x4x4 k-point grid can be replaced
> with 4x4x1, because sampling the Brillouin zone along the direction normal
> to the surface is useless, if the periodic replica of the slab are
> separated by vacuum space (z direction is in this case a “non-periodic”
> direction, sampled with a periodic supercell).
>
> Giovanni
>
>
> --
>
> Giovanni Cantele, PhD
> CNR-SPIN
> c/o Dipartimento di Fisica
> Universita' di Napoli "Federico II"
> Complesso Universitario M. S. Angelo - Ed. 6
> Via Cintia, I-80126, Napoli, Italy
> e-mail: giovanni.cantele at spin.cnr.it
> Phone: +39 081 676910
> Skype contact: giocan74
>
> ResearcherID: http://www.researcherid.com/rid/A-1951-2009
> Web page: http://people.na.infn.it/~cantele
>
>
>
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