Dear Tamas and Kumar
only to add a few words to the (very pertinent) Tamas' reply:
1) Semiconductor surfaces can undergoes very complex reconstruction
patterns (the 7x7 Si(111) reconstruction being likely the most famous
case). You will not find complex reconstructions by simply cutting and
relaxing the slabs, and it is best to search in experimental
literature, in order not to waste time simulating unphysical systems
which are never going to converge.
2) Dangling bonds can be also created inside crystals (e.g, by atomic
vacancies). It is not clear what kind of dangling bond you are
referring to.
3) There is an additional problem when you want to simulate some
process happening on the surface (e.g., adsorptions of molecules).
Various strategies can be used, including the saturation of dangling
bonds on one side of the slab by "pseudohydrogen atoms" having
fractional charge mimicking that of the pristine bond broken by
cleavage.
This said, you should "make an educated guess" [cit. John Malkovich
:-)] and ask less general questions which may (or may not) generate
more useful answers.
HTH
Giuseppe
Quoting Tamas Karpati <[email protected]>:
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
<[email protected]> 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
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
https://lists.quantum-espresso.org/mailman/listinfo/users
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
https://lists.quantum-espresso.org/mailman/listinfo/users
GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <[email protected]>
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
https://lists.quantum-espresso.org/mailman/listinfo/users
