<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On 23 Jun 2014, at 18:44, David Foster <<a href="mailto:davidfoster751@yahoo.com">davidfoster751@yahoo.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">Dear Users<br>Apologize for this simple question. Please guide me:<br>I have used Si bulk for calculating its band gap. I used two approaches for doing it. One with ibrav=2 and working with primitive cell, and the other with ibrav=0<br>and working with its conventional cell.<br>For primitive I used following high-symmetric points:<br>G: 0.0 0.0 0.0<br>X: 0.5 0.0 0.5<br>L: 0.5 0.5 0.5<br>K: 0.375 0.375 0.75<br><br>and for conventional:<br>G: 0.0 0.0 0.0<br>X: 0.5 0.0 0.0<br>M: 0.5 0.5 0.0<br>R: 0.5 0.5 0.5<br><br>I got two different band structures.Conventional cell can be supposed as a supercell of its primitive, so, in the case of conventional, I thought I get only folded<br>band structure. I have attached them.<br><br>Regards<br><br>David Foster<br><br>Ph.D. Student of Chemistry<span><conventional_BS.gif></span><span><primitive_BS.gif></span>_______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://pwscf.org/mailman/listinfo/pw_forum</blockquote></div><div><br></div><div>dear David,</div><div><br></div><div>what you should get is not the same band structure, but a band structure providing exactly the</div><div>same physical properties. Of course, because the simple cubic unit cell is 4 times larger than the ccc</div><div>unit cell, its BZ has a volume 4 time smaller. That is, the 4 occupied</div><div>bands of the ccc unit cell become 16 occupied bands in the sc cell, which is the folding you’re speaking about.</div><div>This being said, the folding is not that trivial as when you simply switch from a cubic crystal with side a to a cubic crystal with side 4a, because</div><div>in your case also the shapes of the BZs are different.</div><div><br></div><div>I’m not really convinced about the band structure of the simple cubic structure is correct. What are the basis atoms you use in that case?</div><div><br></div><div><br></div><div>I hope that I did not make any mistake, but for reference I attach the same calculation (path are just a little bit simpler than yours), were you can follow</div><div>the band “folding”, e.g. along the G-X direction, and recognise, in both cases, the LDA indirect (~0.5 eV) and direct (~2.5 eV) Si band gap. In your</div><div>conventional (simple cubic lattice) plot, I cannot see for example the direct gap at G</div><div>I also attach an image showing the two BZs.</div><div><br></div><div>Hope this helps,</div><div><br></div><div> Giovanni</div><div><br></div><div><br></div><div><img apple-inline="yes" id="FF290A9A-B3E8-4211-AE46-3E411D62AB68" height="729" width="1032" apple-width="yes" apple-height="yes" src="cid:FB024CD4-D530-43A7-A1E5-1D9A03A0019B"></div><div><br></div><div><br></div><div><br></div><div><img apple-inline="yes" id="65892922-41BB-417D-8988-A59514B9BF44" height="563" width="552" apple-width="yes" apple-height="yes" src="cid:3852846E-DD3E-495B-A37A-CE82C94BFA2C"></div><div><br></div><div apple-content-edited="true">
-- <br><br>Giovanni Cantele, PhD<br>CNR-SPIN<br>c/o Dipartimento di Fisica<br>Universita' di Napoli "Federico II"<br>Complesso Universitario M. S. Angelo - Ed. 6<br>Via Cintia, I-80126, Napoli, Italy<br>e-mail: <a href="mailto:giovanni.cantele@spin.cnr.it">giovanni.cantele@spin.cnr.it</a><br>Phone: +39 081 676910<br>Skype contact: giocan74<br><br>ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009">http://www.researcherid.com/rid/A-1951-2009</a><br>Web page: <a href="http://people.na.infn.it/~cantele">http://people.na.infn.it/~cantele</a><br>
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