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 -- ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ?gaux en droits. Les distinctions sociales ne peuvent ?tre fond?es que sur l'utilit? commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la libert?, la propri?t?, la s?ret? et la r?sistance ? l'oppression. ******************************************************** ? ?Giuseppe Mattioli ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?CNR - ISTITUTO DI STRUTTURA DELLA MATERIA ? ? ?v. Salaria Km 29,300 - C.P. 10 ? ? ? ? ? ? ? ? ? ?I 00015 - Monterotondo Stazione (RM) ? ? ? ? ? ? ?Tel + 39 06 90672836 - Fax +39 06 90672316 ? ? ? ?E-mail: <giuseppe.mattioli at ism.cnr.it>
