Dear All, <br>Thank you very much for your help.<br>Please let me ask my final question w.r.t DOS shift of energy of SiC supercell.<br>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.<br>
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.<br>
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.<br>As you know, I need to find a reference for my further calculations.<br>
I really really appreciate your help.<br>Yours<br>P Shok<br><pre>Dear All,<br> let me clarify a little point.<br> The potential (hence the eigenvalues) of any calculation in <br>periodic boundary conditions, not just a plane-wave one , is defined up <br>
to an additive constant... In a finite system a reeference can be fixed <br>by requiring that the potential goes to zero at infinite distance but in <br>a periodic system this cannot be defined as there is nothing like a <br>
point at infinite distance ...<br> In most codes (and in pw.x) this arbitrary constant is fixed by <br>setting the average electrostatic potential to zero. If the supercell is <br>properly constructed, with the properly scaled dimensions, the same <br>
cutoff, equivalent k-points etc.. the two calculations should be <br>equivalent and the eigenvalues should be exactly mapped, the average <br>potential should be the same, the total energy should be an exact <br>multiple of the one of the original cell.<br>
<br>If this does not happen one is NOT doing the supercell calculation properly.<br><br>So if when doing a supercell you do not get the scaled result check your <br>system definition.<br><br>stefano<br><br><br>On 04/04/2011 11:46 AM, Giuseppe Mattioli wrote:<br>
><i> Dear all<br></i>><i> I suspect that a a little misunderstanding has been going on here...<br></i>><i> I try to reformulate the question: in the case of a 8-atoms cell the valence<br></i>><i> band maximum (VBM) has been found around 10 eV and the conduction band<br>
</i>><i> minimum (CBM) around 12 eV with a 2 eV band gap. So there is a shift in the<br></i>><i> position of VBM and CBM when calculated in a 72-atoms supercell, but the band<br></i>><i> gap value is the same. Then, the answer to P Shok could be: plane wave<br>
</i>><i> pseudopotential calculations do not ensure an universal alignment of<br></i>><i> eigenvalues. You should use some "internal" reference, like the 1s eigenvalue<br></i>><i> of an He atom, in your cell; or trust that the two VBM values are aligned "de<br>
</i>><i> facto".<br></i>><i> HTH<br></i>><i><br></i>><i> Giuseppe<br></i>><i><br></i>><i> On Monday 04 April 2011 09:39:18 Gabriele Sclauzero wrote:<br></i>>><i> Yes,<br></i>>><i><br></i>>><i> I perfectly agree with Eyvaz! Moreover the experimental bandgap is<br>
</i>>><i> underestimated within LDA or GGA, and for 4H-SiC you would get about<br></i>>><i> 2.2-2.3 eV with LDA.<br></i>>><i><br></i>>><i> How do you measure the bandgap? Beware that in the fundamental cell the<br>
</i>>><i> bandgap is not a direct one, but it might become such when you use a<br></i>>><i> supercell (because of the refolding of the Brillouin zones).<br></i>>><i><br></i>>><i><br></i>>><i> HTH<br>
</i>>><i><br></i>>><i> GS<br></i>>><i><br></i>>><i> Il giorno 03/apr/2011, alle ore 10.59, Eyvaz Isaev ha scritto:<br></i>>>><i> Hi,<br></i>>>><i><br></i>>>>><i> The bandgap of 8-atoms is around 10-12 eV,<br>
</i>>>>><i> but that of 72-atom is around 5-7 eV.<br></i>>>><i> This is unbelievable, as SiC is an indirect band gap semiconductor and<br></i>>>><i> the bandgap is around (2.5 - 3)eV depending on crystal modification<br>
</i>>>><i> (3C-SiC, 2H-SiC; 4H-SiC; 6H-SiC, etc.).<br></i>>>><i> See <a href="http://www.matprop.ru/SiC_bandstr">http://www.matprop.ru/SiC_bandstr</a><br></i>>>><i><br></i>>>><i> So, check carefully your input/output files.<br>
</i>>>><i><br></i>>>><i> Bests,<br></i>>>><i> Eyvaz.<br></i>>>><i><br></i>>>><i> -------------------------------------------------------------------<br></i>>>><i> Prof. Eyvaz Isaev,<br>
</i>>>><i> Department of Physics, Chemistry, and Biology (IFM), Linkoping<br></i>>>><i> University, Sweden Theoretical Physics Department, Moscow State Institute<br></i>>>><i> of Steel& Alloys, Russia, <a href="http://www.democritos.it/mailman/listinfo/pw_forum">isaev at ifm.liu.se</a>, <a href="http://www.democritos.it/mailman/listinfo/pw_forum">eyvaz_isaev at yahoo.com</a><br>
</i>>>><i><br></i>>>><i><br></i>>>><i> From: pari shok<<a href="http://www.democritos.it/mailman/listinfo/pw_forum">parishok at gmail.com</a>><br></i>>>><i> To: <a href="http://www.democritos.it/mailman/listinfo/pw_forum">pw_forum at pwscf.org</a><br>
</i>>>><i> Sent: Sun, April 3, 2011 1:34:51 AM<br></i>>>><i> Subject: [Pw_forum] shift of energy<br></i>>>><i><br></i>>>><i> Dear Paolo,<br></i>>>><i> DOS of 72-atom SiC shows a shift of energy with respect to 8-atom SiC.The<br>
</i>>>><i> bandgap of 8-atoms is around 10-12 eV, but that of 72-atom is around 5-7<br></i>>>><i> eV. Would you please help me to understand this shift.<br></i>>>><i><br></i>>>><i> Thanks again.<br>
</i>>>><i> P Shok<br></i>>>><i> UMD<br></i>>>><i><br></i>>>><i> On Apr 1, 2011, at 21:36 , pari shok wrote:<br></i>>>>><i> The DOS of 72-atom SiC (supercell) shows a shift of energy.<br>
</i>>>><i> a shift with respect to what?<br></i>>>><i> ---<br></i>>>><i> Paolo Giannozzi, Dept of Chemistry&Physics&Environment,<br></i>>>><i><br></i>>>><i> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy<br>
</i>>>><i> Phone +39-0432-558216, fax +39-0432-558222<br></i>>>><i><br></i>>>><i> _______________________________________________<br></i>>>><i> Pw_forum mailing list<br></i>>>><i> <a href="http://www.democritos.it/mailman/listinfo/pw_forum">Pw_forum at pwscf.org</a><br>
</i>>>><i> <a href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</a><br></i>>><i> § Gabriele Sclauzero, EPFL SB ITP CSEA<br></i>>><i> PH H2 462, Station 3, CH-1015 Lausanne<br>
</i>><i><br></i>><i><br></i></pre><br>