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

[bramha pandey]

 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.

Thanks and Regards
Bramha Prasad Pandey
GLA University, Mathura.
INDIA.

On Wed, Mar 28, 2012 at 4:16 PM, Giuseppe Mattioli <
giuseppe.mattioli at ism.cnr.it> wrote:

>
> 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
> > >
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> > > --
> > >
> > > ----------------------------------------------------------------------
> > > Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
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-- 
Thanks and Regards
Bramha Prasad Pandey
GLA University, Mathura.
INDIA.
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