[Pw_forum] why I can not get the correct band gap of TiO2?

[#TAO JUNGUANG#]

Very thanks for your help. I have revised my Input file as you said, but from the output file, 
 
........
End of band structure calculation
          k = 0.0000 0.0000 0.0000     band energies (ev):
   -48.0496 -48.0314 -24.6221 -24.6221 -24.5729 -24.5342 -24.4355 -24.4355
   -10.8942  -9.9348  -9.1108  -9.1108   1.4602   1.7866   2.9518   3.3254
     3.3254   4.5197   4.5197   4.9432   5.9659   6.2973   6.2973   6.9316
from the output file, I think the band gap is (1.4602-(-9.1108)=10.571eV),first, am I right getting band gap like this? Then, if that is the case, the band gap is very large comparing with the experiment value 3.2eV. (Additionally, other people hav givin a value of 2.0eV using pwscf, which is more reasonable than mine).
 
Yes, maybe pwscf is not the best way to get the band gap of TiO2, however my results is very strange, so, I just want to figure out what wrong . 

Thanks!
________________________________

From: pw_forum-admin at pwscf.org on behalf of Eyvaz Isaev
Sent: Wed 10/26/2005 8:38 PM
To: pw_forum at pwscf.org
Subject: Re: [Pw_forum] why I can not get the correct band gap of TiO2?



Hi,

First, some technical comments.
>
>  ecutwfc=16,

It looks very small, use 30Ry. Besides add ecutrho,
say, 300Ry, for  the augmented charge.

>
> &IONS
>
>   ion_dynamics='bfgs',
>
>   upscale=10
>
>   /

For the band gap problem you do not need ionic
dynamics.


>
> K_POINTS
>
> 36
K-points you used looks strange. How did you generate
these points? They have equal weight and seem to be
k-points along high symmetry directions.
Use K_POINTS {automatic}.

Now about your question. DFT is known to give
underestimated and sometime wrong band gap for
semiconductors and insulators. You can improve it
using the LDA+U approximation.  
As far as I am aware Exact Exchange is still in
development in Espresso.

Bests,
Eyvaz.


               
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