[Pw_forum] shift of energy

[Giuseppe Mattioli]

Dear all
I suspect that a a little misunderstanding has been going on here...
I try to reformulate the question: in the case of a 8-atoms cell the valence 
band maximum (VBM) has been found around 10 eV and the conduction band 
minimum (CBM) around 12 eV with a 2 eV band gap. So there is a shift in the 
position of VBM and CBM when calculated in a 72-atoms supercell, but the band 
gap value is the same. Then, the answer to P Shok could be: plane wave 
pseudopotential calculations do not ensure an universal alignment of 
eigenvalues. You should use some "internal" reference, like the 1s eigenvalue 
of an He atom, in your cell; or trust that the two VBM values are aligned "de 
facto".
HTH

Giuseppe

On Monday 04 April 2011 09:39:18 Gabriele Sclauzero wrote:
> Yes,
>
>    I perfectly agree with Eyvaz! Moreover the experimental bandgap is
> underestimated within LDA or GGA, and for 4H-SiC you would get about
> 2.2-2.3 eV with LDA.
>
>    How do you measure the bandgap? Beware that in the fundamental cell the
> bandgap is not a direct one, but it might become such when you use a
> supercell (because of the refolding of the Brillouin zones).
>
>
> HTH
>
> GS
>
> Il giorno 03/apr/2011, alle ore 10.59, Eyvaz Isaev ha scritto:
> > Hi,
> >
> > >The bandgap of 8-atoms is around 10-12 eV,
> > >but that of 72-atom is around 5-7 eV.
> >
> > This is unbelievable, as SiC is an indirect band gap semiconductor and
> > the bandgap is around (2.5 - 3)eV depending on crystal modification
> > (3C-SiC, 2H-SiC; 4H-SiC; 6H-SiC, etc.).
> > See http://www.matprop.ru/SiC_bandstr
> >
> > So, check carefully your input/output files.
> >
> > Bests,
> > Eyvaz.
> >
> > -------------------------------------------------------------------
> > Prof. Eyvaz Isaev,
> > Department of Physics, Chemistry, and Biology (IFM), Linkoping
> > University, Sweden Theoretical Physics Department, Moscow State Institute
> > of Steel & Alloys, Russia, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
> >
> >
> > From: pari shok <parishok at gmail.com>
> > To: pw_forum at pwscf.org
> > Sent: Sun, April 3, 2011 1:34:51 AM
> > Subject: [Pw_forum] shift of energy
> >
> > Dear Paolo,
> > DOS of 72-atom SiC shows a shift of energy with respect to 8-atom SiC.The
> > bandgap of 8-atoms is around 10-12 eV, but that of 72-atom is around 5-7
> > eV. Would you please help me to understand this shift.
> >
> > Thanks again.
> > P Shok
> > UMD
> >
> > On Apr 1, 2011, at 21:36 , pari shok wrote:
> > > The DOS of 72-atom SiC (supercell) shows a shift of energy.
> >
> > a shift with respect to what?
> > ---
> > Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
> >
> > Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> > Phone +39-0432-558216, fax +39-0432-558222
> >
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
>
> § Gabriele Sclauzero, EPFL SB ITP CSEA
>    PH H2 462, Station 3, CH-1015 Lausanne



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   Giuseppe Mattioli                            
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