[QE-users] Convergence tests for slabs

[Giovanni Cantele]

Strictly speaking, you should get convergence of the clean slab first,
concerning both the slab thickness and the vacuum. Consider that thin
slabs might suffer from "quantum confinement" effects, so it is important
to look at the changes of the electronic properties if you are interested
in accurately reproducing them, especialy as far as 'bulk' states are
concerned.

Once you place a molecule on the slab, you should take care of the fact
that the molecule "thickness" in the direction perpendicular to the slab
actually corresponds to decreasing the vacuum width. As such, you should
add to the vacuum this thickness, even approximately, to avoid what you
mention.

Also consider that, if you want to get rid of effects of the slab asymmetry
once the molecule has been placed on the slab, you should include dipole
corrections. The alternative is to build a symmetric slab+molecule system,
where the molecule is on both sides of the slab, which however is much more
computationally expensive, because it requires a large unit cell size in
the direction perpendicular to the slab.

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 <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home


Il giorno lun 19 dic 2022 alle ore 10:32 Léon Luntadila Lufungula <
Luntadilatiti at hotmail.com> ha scritto:

> Dear QE users,
>
>
>
> I’m creating a slab for studying adsorption of small molecules (around 6-8
> atoms in length) and want to converge both the slab thickness and vacuum
> width. However, I don’t fully understand how to do this properly… For the
> convergence of the slab thickness, I was thinking about taking slabs of
> different thickness and calculating the surface energy and look at when the
> difference between two consecutive surface energies is smaller than 1
> kcal/mol (i.e. chemical accuracy), but I’ve also heard that you can look at
> the difference in energy when adding a layer and converging this energy
> difference to chemical accuracy. Any suggestions on which is the better
> option? For the vacuum width, I’m confused if you should converge this
> value on a clean slab or on a slab with a model compound adsorbed onto it.
> I’ve been told you should do this on a clean slab, but I would expect that
> once an adsorbate is bonded onto the surface, the end of the adsorbate
> might be close enough to the next layer to interact with it, which would
> make the convergence test obsolete…
>
>
>
> I would be most thankful if someone could tell me the correct procedure in
> this situation!
>
>
>
> Thanks in advance and kind regards,
>
> Léon Luntadila Lufungula
>
> Structural Chemistry group
>
> University of Antwerp
>
>
>
>
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