<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">You are luck, because I already answered a similar question some years ago, so I’ve just needed the time</div><div class="">to find useful plots somewhere in my archives!!!</div><div class=""><br class=""></div><div class=""><br class=""></div>Your band structure is likely to be correct. One more thing you should consider when you<div class="">use non primitive cells is band folding. The simple cubic (SC) cell has a volume that is 4 times</div><div class="">that of the FCC cell, indeed you use 4 Si-Si formula units. That means that you get, for each fixed k,</div><div class="">4 times the number of bands with respect to the FCC calculation but the Brillouin zone (BZ) is 4 time smaller.</div><div class=""><br class=""></div><div class="">See the 1st attachment: the BZ of the SC crystal is contained WITHIN the BZ of the FCC crystal. The X point</div><div class="">at the boundary of the LARGER BZ has (in units 2*pi/a) coordinates (1,0,0). On the other hand, in the SMALLER</div><div class="">BZ the boundary along the <100> direction is (in the SAME units) at (1/2,0,0). That means that the 1st X_FCC point is,</div><div class="">in the SC system, equivalent to Gamma because they differ by a reciprocal vector of the SC system, (1, 0, 0).</div><div class=""><br class=""></div><div class="">In FCC the conduction band minimum is found at about 0.85 G-X_FCC segment.</div><div class=""><br class=""></div><div class="">In SC, the bands along G - X_FCC / 2 remain the same, whereas the X_FCC / 2 - X_FCC is folded back</div><div class="">with a sign inversion:</div><div class="">X_FCC / 2 equivalent to X_SC = (1/2,0,0)</div><div class="">X_FCC equivalent to Gamma_SC = (0,0,0)</div><div class=""><br class=""></div><div class="">The conduction band minimum is thus obtained at (1-0.85) Gamma_SC - X_SC segment, so at about 0.15 * (2*pi/a, 0, 0)</div><div class=""><br class=""></div><div class="">If you look at the second attachment, this is exactly what you obtain, what was missing in your calculation is that you did not</div><div class="">include, in the SC calculation, the Gamma_SC - X_SC direction.</div><div class=""><br class=""></div><div class="">Giovanni</div><div class=""><br class=""></div><div class="">PS beware: in the second attachment the FCC and SC band structures of Si are compared with each other, but the X point</div><div class="">in the left graph IS NOT the same of the X point in the right one, one is X_SC = (pi/a, 0, 0), the other X_FCC = (2*pi/a, 0, 0)</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><div class=""><img apple-inline="yes" id="CC46DC04-7D59-4667-9E0E-FDFE4B28B7DC" src="cid:D65B08B4-9A00-45B2-9B50-2DDF98CF4E98" class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><img apple-inline="yes" id="BD08F8A4-6136-4CC2-A74A-67867FD32742" src="cid:ACA7DA39-1E51-468F-9820-B3E5A5849C7C" class=""></div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On 17 Mar 2017, at 17:55, phs157144 <<a href="mailto:phs157144@iitd.ac.in" class="">phs157144@iitd.ac.in</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">Thank you so much Giovanni for your prompt response.<br class="">I repeated the calculations using unshifted k-point grid with the <br class="">k-path<br class="">gamma-R-X-M-gamma as you instructed and got better result.( <br class=""><a href="https://www.dropbox.com/s/a3przkx0cl193ao/Si111r.pdf?dl=0" class="">https://www.dropbox.com/s/a3przkx0cl193ao/Si111r.pdf?dl=0</a> ). But still <br class="">the band gap seems to be direct.<br class=""><br class=""><br class="">On 17.03.2017 16:37, Giovanni Cantele wrote:<br class=""><blockquote type="cite" class="">Before answering your question about gaps, two minor remarks:<br class=""><br class="">i) you’re using norm conserving pseudo potential, you do not need to<br class="">set ecutrho to a value<br class="">larger than 4*ecutwfc, use the default instead<br class=""><br class="">ii) you are representing the Si crystal with a simple cubic supercell.<br class="">When you calculate the<br class="">band structure, you should use special points of the Brillouin zone of<br class="">a simple cubic crystal,<br class="">whereas as far as I understand you are using the special points of the<br class="">Brillouin zone of a<br class="">cubic fcc crystal. Of course you don’t get anything wrong (you can<br class="">diagonalise the Hamiltonian<br class="">at whatever point in k space you want), but this is not the way in<br class="">which the electronic structure of<br class="">a cubic crystal is usually represented.<br class=""><br class=""><br class="">This being said, you can easily understand that the DOS you compute is<br class="">INCOMPATIBLE<br class="">with the band structure you show, because slightly above -2 eV you get<br class="">an energy gap<br class="">in the DOS, whereas you see electronic states in the band structure.<br class="">So, either you are calculating<br class="">the DOS and band structure of two different systems, or the DOS<br class="">derives from a calculation where<br class="">the Brillouin zone has not been correctly sampled. The latter<br class="">explanation applies to your case.<br class="">Indeed, in the nscf calculation preceding the dos.x run you use<br class="">K_POINTS {automatic}<br class=""> 8 8 8 1 1 1<br class="">that is, a shifted k-point grid that, as such, does not include gamma <br class="">point.<br class="">From the band structure you see that the electronic states in the<br class="">energy window -2 : -1 eV are just<br class="">localised around Gamma, so if you get rid of Gamma point you just<br class="">obtain nothing in the DOS.<br class=""><br class="">The shifted grid would return the correct result if you make it finer,<br class="">but it is anyway better to use an<br class="">unshifted grid.<br class=""><br class=""><br class="">Giovanni<br class=""><br class=""><br class=""><blockquote type="cite" class="">On 17 Mar 2017, at 07:35, phs157144 <<a href="mailto:phs157144@iitd.ac.in" class="">phs157144@iitd.ac.in</a>> wrote:<br class=""><br class=""><br class="">Hi every one ,<br class=""> I am new to Quantum Espresso and using v.6.0 (svn rev. 13079). I <br class="">was<br class="">doing the band structure calculation of Si using supercell and <br class="">obtained<br class="">the electronic band structure and DOS (<br class=""><a href="https://www.dropbox.com/s/shh289r33uod32e/Si111.pdf?dl=0" class="">https://www.dropbox.com/s/shh289r33uod32e/Si111.pdf?dl=0</a> ). The result<br class="">is showing two band gaps,one at the fermi level and the other one is<br class="">near -2 eV . The band gap at the fermi level seems to be direct. Am I<br class="">doing wrong? I have attached my input file:<br class=""><a href="https://www.dropbox.com/s/vqml9lq6v4pnr1w/Si111.sh?dl=0" class="">https://www.dropbox.com/s/vqml9lq6v4pnr1w/Si111.sh?dl=0</a><br class="">Please comment.<br class=""><br class="">--<br class="">Ruhul Amin<br class="">M.Sc,physics<br class="">IIT Delhi<br class="">_______________________________________________<br class="">Pw_forum mailing list<br class="">Pw_forum@pwscf.org<br class="">http://pwscf.org/mailman/listinfo/pw_forum<br class=""><br class=""></blockquote><br class="">--<br class=""><br class="">Giovanni Cantele, PhD<br class="">CNR-SPIN<br class="">c/o Dipartimento di Fisica<br class="">Universita' di Napoli "Federico II"<br class="">Complesso Universitario M. S. Angelo - Ed. 6<br class="">Via Cintia, I-80126, Napoli, Italy<br class="">e-mail: <a href="mailto:giovanni.cantele@spin.cnr.it" class="">giovanni.cantele@spin.cnr.it</a><br class="">Phone: +39 081 676910<br class="">Skype contact: giocan74<br class=""><br class="">ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009" class="">http://www.researcherid.com/rid/A-1951-2009</a><br class="">Web page: <a href="http://people.na.infn.it/~cantele" class="">http://people.na.infn.it/~cantele</a><br class=""><br class=""><br class=""><br class="">_______________________________________________<br class="">Pw_forum mailing list<br class=""><a href="mailto:Pw_forum@pwscf.org" class="">Pw_forum@pwscf.org</a><br class="">http://pwscf.org/mailman/listinfo/pw_forum<br class=""></blockquote><br class="">-- <br class="">Ruhul Amin<br class="">M.Sc,physics<br class="">IIT Delhi<br class=""><br class="">_______________________________________________<br class="">Pw_forum mailing list<br class=""><a href="mailto:Pw_forum@pwscf.org" class="">Pw_forum@pwscf.org</a><br class="">http://pwscf.org/mailman/listinfo/pw_forum</div></div></blockquote></div><br class=""><div class="">
-- <br class=""><br class="">Giovanni Cantele, PhD<br class="">CNR-SPIN<br class="">c/o Dipartimento di Fisica<br class="">Universita' di Napoli "Federico II"<br class="">Complesso Universitario M. S. Angelo - Ed. 6<br class="">Via Cintia, I-80126, Napoli, Italy<br class="">e-mail: <a href="mailto:giovanni.cantele@spin.cnr.it" class="">giovanni.cantele@spin.cnr.it</a><br class="">Phone: +39 081 676910<br class="">Skype contact: giocan74<br class=""><br class="">ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009" class="">http://www.researcherid.com/rid/A-1951-2009</a><br class="">Web page: <a href="http://people.na.infn.it/~cantele" class="">http://people.na.infn.it/~cantele</a><br class="">
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