[QE-users] Dangling bonds with CdS

[Tamas Karpati]

Dear Dr. Kumar,

I guess dangling bonds are written about in DFT textbooks of the
physicists' style
(in quantum chemistry, ie. clusters rather than crystals, it is less
of a problem).

The typical problem is that you cut chemical bonds when cleave the
crystal (to get a slab).
In case of bonds originally dominated by ionic (Coulomb) forces you
may still have a closed
shell system (ie. just paired electrons) after the cleavage. As for
the more covalent
bonds, radicals are generated and such high multiplicity electronic
states (large
magnetizations in terms of QE/PW.x inputs) reorganize to the more stable closed
shell systems by changing geometry (you need to reoptimize their geometry) and
simultaneously forming new bonds. This way every few surface atom
pairs get closer
and such bonds form, ie. bonds that were dangling after you made the cut are now
in covalent bonds again (no dangling anymore).
This you can model by reoptimizing with low (not sure but probably
zero) magnetization.

CdS has strong bonds carrying both ionic (maybe less) and covalent nature
(more of the latter). Depending on the cleaving plane you applied to
the crystal,
you have a high chance to see new Cd-S bonds form. Less probable is that you'll
have Cd-Cd bonds or -S-S- bridges but these are also possible if your surface
atoms are situated so.

Another way of treating such systems is to keep the structure as you have cut
from the crystal and apply a high starting_magnetization in a PW/relax job.

Be careful, though, as the chemistries you describe by the above two methods
(reorganization vs. high magnetization) are living in two distant Universes.

I hope this helps,
  Tamas

On Thu, Nov 12, 2020 at 8:13 AM Dr. SUNIL KUMAR <suniliitd14 at gmail.com> wrote:
>
> Dear QE developers and Users.
> I am struggling to carry out a DFT simulation of CdS with dangling bonds using Quantum Espresso DFT packages. I am unable to understand the phenomena of Dangling bond and its implementation in QE DFT simulation. I would like to request to you all, kindly suggest me some tutorial and sample QE scripts for CdS with Dangling bonds. I will be grateful to you.
>
> Thanks
> With regards
> SUNIL
> Dr. Sunil Kumar
> Ph.D (Chemical Engg. IIT Delhi)
> M.Tech (Chemical Engg. IIT Delhi)
> B.Tech (Chemical Engg. IET-CSJMU Kanpur)
> Scientist-C and Assistant Professor
> CSIR-National Metallurgical Laboratory Jamshedpur-831007
> http://www.nmlindia.org/
> https://scholar.google.co.in/citations?user=OchYqugAAAAJ&hl=en&oi=sra
>
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