<div>Dear Professor Giannozi and Marzari</div>
<div> </div>
<div> Many thanks for your helpful comments.<br><br></div>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">david grifith wrote:<br><br>> yet I have not understood where is selected as zero energy<br>> in the Bulk graphite by QE ? or generally where is selected<br>
> as zero energy in QE calculations?<br><br>nothing is selected as "zero energy". One-electron energies,<br>including the Fermi energy, are calculated with respect to some<br>arbitrary reference energy (see the paper cited in the first of<br>
the following links). If you want to set your zero of the energy<br>at the Fermi energy, you can do it in your preferred plotting<br>program. If instead you want to know how this arbitrary reference<br>level is calculatd in the code, see the notes in the last link.<br>
<br><a href="http://www.democritos.it/pipermail/pw_forum/2004-August/001240.html" target="_blank">http://www.democritos.it/pipermail/pw_forum/2004-August/001240.html</a><br><a href="http://www.democritos.it/pipermail/pw_forum/2007-April/006351.html" target="_blank">http://www.democritos.it/pipermail/pw_forum/2007-April/006351.html</a><br>
<a href="http://www.democritos.it/pipermail/pw_forum/2009-June/013231.html" target="_blank">http://www.democritos.it/pipermail/pw_forum/2009-June/013231.html</a><br><a href="http://www.democritos.it/pipermail/pw_forum/2009-June/013232.html" target="_blank">http://www.democritos.it/pipermail/pw_forum/2009-June/013232.html</a><br>
<br>P.<br>---<br>Paolo Giannozzi, Dept of Chemistry&Physics, Univ. Udine<br>via delle Scienze 208, 33100 Udine, Italy<br>Phone +39-0432-558216, fax +39-0432-558222<br><br><br><br><br>On this thread - let me also stress that lacking an absolute<br>
scale of energies is an intrinsic feature of calculations<br>done in periodic-boundary conditions, and not a feature or a<br>defect of q-e. It's universal to any solid-state code (abinit,<br>vasp, castep...). it's also very instructive to understand<br>
why it happens, and an extensive discussion of the issues related<br>to finding the position of the vacuum level and the electrostatic<br>averaging technique (Baldereschi et al Phys. Rev. Lett. 61,<br>734 (1988)) can be found in Caspar Fall's thesis<br>
<a href="http://library.epfl.ch/en/theses/?nr=1955" target="_blank">http://library.epfl.ch/en/theses/?nr=1955</a> or in<br>Singh-Miller et al Phys. Rev. B 80, 235407 (2009), while the<br>issues related to electrostatics in periodic-boundary conditions<br>
are discussed in Dabo et al Phys. Rev. B 77 115139 (2008).<br><br>nicola<br>?<br><br>--<br>---------------------------------------------------------------------<br>Prof Nicola Marzari Department of Materials Science and Engineering<br>
13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA<br>tel 617.4522758 fax 2586534 <a href="mailto:marzari@mit.edu">marzari@mit.edu</a> <a href="http://quasiamore.mit.edu/" target="_blank">http://quasiamore.mit.edu</a><br>
<br><br>--<br>Sincerely Yours<br>David G.<br>JCU<br></blockquote></div>