[Pw_forum] Electronic band structure calculation of Si using supercell

[Giovanni Cantele]

Before answering your question about gaps, two minor remarks: 

i) you’re using norm conserving pseudo potential, you do not need to set ecutrho to a value
larger than 4*ecutwfc, use the default instead

ii) you are representing the Si crystal with a simple cubic supercell. When you calculate the
band structure, you should use special points of the Brillouin zone of a simple cubic crystal,
whereas as far as I understand you are using the special points of the Brillouin zone of a
cubic fcc crystal. Of course you don’t get anything wrong (you can diagonalise the Hamiltonian
at whatever point in k space you want), but this is not the way in which the electronic structure of
a cubic crystal is usually represented.


This being said, you can easily understand that the DOS you compute is INCOMPATIBLE
with the band structure you show, because slightly above -2 eV you get an energy gap
in the DOS, whereas you see electronic states in the band structure. So, either you are calculating
the DOS and band structure of two different systems, or the DOS derives from a calculation where
the Brillouin zone has not been correctly sampled. The latter explanation applies to your case.
Indeed, in the nscf calculation preceding the dos.x run you use
K_POINTS {automatic}
  8 8 8 1 1 1
that is, a shifted k-point grid that, as such, does not include gamma point.
>From the band structure you see that the electronic states in the energy window -2 : -1 eV are just
localised around Gamma, so if you get rid of Gamma point you just obtain nothing in the DOS.

The shifted grid would return the correct result if you make it finer, but it is anyway better to use an
unshifted grid.


Giovanni


> On 17 Mar 2017, at 07:35, phs157144 <phs157144 at iitd.ac.in> wrote:
> 
> 
> Hi every one ,
>    I am new to Quantum Espresso and using v.6.0 (svn rev. 13079). I was 
> doing the band structure calculation of Si using  supercell and obtained 
> the electronic band structure and DOS ( 
> https://www.dropbox.com/s/shh289r33uod32e/Si111.pdf?dl=0 ). The result 
> is showing two band gaps,one at the fermi level and the other one is 
> near -2 eV . The band gap at the fermi level  seems to be direct. Am I 
> doing wrong? I have attached my input file:
> https://www.dropbox.com/s/vqml9lq6v4pnr1w/Si111.sh?dl=0
> Please comment.
> 
> -- 
> Ruhul Amin
> M.Sc,physics
> IIT Delhi
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-- 

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