[Pw_forum] Shift of Energy

Stefano de Gironcoli degironc at sissa.it
Mon Apr 4 18:37:07 CEST 2011

```Dear Pari Shok,

a 7 Ry difference is A LOT ! you are not doing things properly.
Then any further analysis is irrelevant

cheers,

stefano

On 04/04/2011 04:32 PM, pari shok wrote:
> Dear All,
> Thank you very much for your help.
> Please let me ask my final question w.r.t DOS shift of energy of SiC
> supercell.
> The total energy of the 8-atom cell of SiC is: -76.276 Ry, while that of
> 72-atom SiC is -694 Ry. Neglecting almost -7 Ry, the total energy is scaling
> accordingly.
> However, I still see the shift of energy in DOS diagram of two slabs (the
> max and min of bandgap shifts, although the gap is almost 2.1 eV for both
> 4H-SiC slabs). I was wondering whether this shift of energy is inevitable or
> it is as the result of my input file.
> I kept k points (w.r.t. crystal), and cut off energy the same for both
> structures. Should I scale the cut off energy as well. If positive, is there
> any relation between scaling the cell and cut off energy.
> As you know, I need to find a reference for my further calculations.
> I really really appreciate your help.
> Yours
> P Shok
>
> Dear  All,
>       let me clarify a little point.
>       The potential (hence the eigenvalues) of  any calculation in
> periodic boundary conditions, not just a plane-wave one , is defined up
> to  an additive constant... In a finite system a reeference can be fixed
> by requiring that the potential goes to zero at infinite distance but in
> a periodic system this cannot be defined as there is nothing like a
> point at infinite distance ...
>      In most codes (and in pw.x) this arbitrary constant is fixed by
> setting the average electrostatic potential to zero. If the supercell is
> properly constructed, with the properly scaled dimensions, the same
> cutoff, equivalent k-points etc.. the two calculations should be
> equivalent and the eigenvalues should be exactly mapped, the average
> potential should be the same, the total energy should be an exact
> multiple of the one of the original cell.
>
> If this does not happen one is NOT doing the supercell calculation properly.
>
> So if when doing a supercell you do not get the scaled result check your
> system definition.
>
> stefano
>
>
> On 04/04/2011 11:46 AM, Giuseppe Mattioli wrote:
>> * 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
> <http://www.democritos.it/mailman/listinfo/pw_forum>, eyvaz_isaev at
> yahoo.com<http://www.democritos.it/mailman/listinfo/pw_forum>
> *>>>*
> *>>>*
> *>>>* From: pari shok<parishok at gmail.com
> <http://www.democritos.it/mailman/listinfo/pw_forum>>
> *>>>* To: pw_forum at pwscf.org
> <http://www.democritos.it/mailman/listinfo/pw_forum>
> *>>>* 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
> *>>>*
> *>>>* 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>
> *>>>* 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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