[Pw_forum] Electronic band structure calculation of Si using supercell

[phs157144]

Thank you so much Giovanni for your prompt response.
I repeated the calculations using unshifted k-point grid  with the 
k-path
gamma-R-X-M-gamma as you instructed and got better result.( 
https://www.dropbox.com/s/a3przkx0cl193ao/Si111r.pdf?dl=0 ). But still 
the band gap seems to be direct.


On 17.03.2017 16:37, Giovanni Cantele wrote:
> 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
> 
> 
> 
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-- 
Ruhul Amin
M.Sc,physics
IIT Delhi