[Pw_forum] Tow question about band gap and occupations= 'smearing or fixed

[Giuseppe Mattioli]

Dear Naji (and Nicola)

> Dear Sir, so what approch should we take to calculate the band gap of
> semiconductor structure in QE...

> > last, of course the dft band gap is fairly useless.

Very old question, old like DFT... If you cannot afford anything but DFT, then you are stuck to DFT 
errors (see, for instance, Paula Mori-Sanchez, Aron J. Cohen, and Weitao Yang, PRL 100, 146401 
(2008)). But DFT band gaps, if not accurate, may be large enough to perform investigations on 
semiconductor defects, interfaces, ... If you are working on unit cells and you need more accurate 
predictions you may consider to perform hybrid exact-exchange calculations, and this is only one of 
the solutions. If you have to deal with larger supercells, you may try DFT+U corrections. Hard to 
say without looking at your system.

Last but not least, Please remember to sign your posts and to provide your scientific affiliation.

HTH

Giuseppe

On Wednesday 28 March 2012 12:20:53 bramha pandey wrote:
> 2012/3/28 Nicola Marzari <nicola.marzari at epfl.ch>
> 
> > There is no harm in using smearing in a semiconductor/insulator (only
> > an increased cost in the calculation).
> > 
> > To get the band gap you need in any case the empty states - but note that
> > unless you know at which k-point the band gap is, you need to do a close
> > examination of the entire band structure.
> > 
> > last, of course the dft band gap is fairly useless.
> 
> Dear Sir, so what approch should we take to calculate the band gap of
> semiconductor structure in QE...Thanks in advanced for your kind response.
> 
> >                        nicola
> > 
> > On 28/03/2012 11:25, sufyan wrote:
> > > Dear quantum espresso user, i study a case with different constrain and
> > > under some constrain this case changes from conductor to semiconductor
> > > and vers-versa and I have a question firstly about "occupations=
> > > 'smearing or fixed '" what can i do with it ? secondly Does the pw.x
> > > give the band gap as numerical results in the out file ? thanks sufyan
> > 
> > naji
> > 
> > > _______________________________________________
> > > Pw_forum mailing list
> > > Pw_forum at pwscf.org
> > > http://www.democritos.it/mailman/listinfo/pw_forum
> > 
> > --
> > 
> > ----------------------------------------------------------------------
> > Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum

-- 
********************************************************
- Article premier - Les hommes naissent et demeurent
libres et ègaux en droits. Les distinctions sociales
ne peuvent être fondèes que sur l'utilitè commune
- Article 2 - Le but de toute association politique
est la conservation des droits naturels et 
imprescriptibles de l'homme. Ces droits sont la libertè,
la propriètè, la sùretè et la rèsistance à l'oppression.
********************************************************

   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM)          
   Tel + 39 06 90672836 - Fax +39 06 90672316    
   E-mail: <giuseppe.mattioli at ism.cnr.it>