Thank you Dear Giuseppe Mattioli and Nicola for your quick reply to let me the use of hybrid exact-exchange calculations to get some what accurate value of band gap energy in quantum espresso.<br><br>Thanks and Regards<br>
Bramha Prasad Pandey<br>GLA University, Mathura.<br>INDIA.<br><br><div class="gmail_quote">On Wed, Mar 28, 2012 at 4:16 PM, Giuseppe Mattioli <span dir="ltr"><<a href="mailto:giuseppe.mattioli@ism.cnr.it">giuseppe.mattioli@ism.cnr.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
Dear Naji (and Nicola)<br>
<div class="im"><br>
> Dear Sir, so what approch should we take to calculate the band gap of<br>
> semiconductor structure in QE...<br>
<br>
> > last, of course the dft band gap is fairly useless.<br>
<br>
</div>Very old question, old like DFT... If you cannot afford anything but DFT, then you are stuck to DFT<br>
errors (see, for instance, Paula Mori-Sanchez, Aron J. Cohen, and Weitao Yang, PRL 100, 146401<br>
(2008)). But DFT band gaps, if not accurate, may be large enough to perform investigations on<br>
semiconductor defects, interfaces, ... If you are working on unit cells and you need more accurate<br>
predictions you may consider to perform hybrid exact-exchange calculations, and this is only one of<br>
the solutions. If you have to deal with larger supercells, you may try DFT+U corrections. Hard to<br>
say without looking at your system.<br>
<br>
Last but not least, Please remember to sign your posts and to provide your scientific affiliation.<br>
<br>
HTH<br>
<br>
Giuseppe<br>
<div><div class="h5"><br>
On Wednesday 28 March 2012 12:20:53 bramha pandey wrote:<br>
> 2012/3/28 Nicola Marzari <<a href="mailto:nicola.marzari@epfl.ch">nicola.marzari@epfl.ch</a>><br>
><br>
> > There is no harm in using smearing in a semiconductor/insulator (only<br>
> > an increased cost in the calculation).<br>
> ><br>
> > To get the band gap you need in any case the empty states - but note that<br>
> > unless you know at which k-point the band gap is, you need to do a close<br>
> > examination of the entire band structure.<br>
> ><br>
> > last, of course the dft band gap is fairly useless.<br>
><br>
> Dear Sir, so what approch should we take to calculate the band gap of<br>
> semiconductor structure in QE...Thanks in advanced for your kind response.<br>
><br>
> > nicola<br>
> ><br>
> > On 28/03/2012 11:25, sufyan wrote:<br>
> > > Dear quantum espresso user, i study a case with different constrain and<br>
> > > under some constrain this case changes from conductor to semiconductor<br>
> > > and vers-versa and I have a question firstly about "occupations=<br>
> > > 'smearing or fixed '" what can i do with it ? secondly Does the pw.x<br>
> > > give the band gap as numerical results in the out file ? thanks sufyan<br>
> ><br>
> > naji<br>
> ><br>
> > > _______________________________________________<br>
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> ><br>
> > --<br>
> ><br>
> > ----------------------------------------------------------------------<br>
> > Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL<br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Thanks and Regards<br>Bramha Prasad Pandey<br>GLA University, Mathura.<br>INDIA.<br><br>