[Wien] band gap problem for optic calculation

[Rocquefelte]

Dear Tomo,

I also noticed such behaviour. In fact, you are using approaches 
allowing to go "beyond DFT" but the correction you are doing is not exact.
Depending of the system you will have the possibility to reproduce the 
experimental band gap using GGA+U (for localized orbitals - 3d, 4f...) 
or using hybrid functional.
In all cases, the fact that you reproduce the band gap doesn't mean that 
the shape of the dielectric function is correctly reproduced, and in 
fact in many cases you have a situation leading to an underestimation of 
the refractive index when the band gap is well-reproduced.

Another problem comes from the definition of the band gap. You should 
compare similar gaps (fundamental, optical band gaps... ).

Regards

Xavier


Le 20/04/2011 10:25, Tomohiko Tasaka a écrit :
> Hi,
>
> I try to calculate optical properties for wide-gap insulators.
> then, I found that the value of refractive index got worse when band
> gap are corrected by GGA+U method or mBJ potential(i.e. underestimate).
> so, is this normal behavior? or, am I wrong??
>
> sorry for the vague question, but any information would be appreciated.
>
> Thanks in advance, with best regards,
> Tomo
>
>
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