[Pw_forum] smaller band gaps

[Lorenzo Paulatto]

In data 23 giugno 2009 alle ore 13:58:38, Naol Regassa  
<greatnaol at gmail.com> ha scritto:
> My main concern is that DFT calculations reduce band gaps
> by 50%.

DFT does not predict band gap at all, for many materials it is related to  
the experimental one, but on assumption can be made a priori on its  
precision. Anyway, you are probably doing the calculation using the  
experimental lattice parameter, if you relax the system to the numerical  
equilibrium lattice parameter, you will probably get a better gap.

> Does this mean that I can add this error to get the correct
> band gap so that any body can understand it? How can one argue about
> this condition?

No, the fact that DFT does not predict the band gap is a well known  
problem of the theory, if you fix it with an empirical parameter it would  
be not only unethical, but also quite suspicious. If you want to have  
accurate band gap you have to use TDDFT or DFT+GW, which are both  
implemented in quantum espresso (I don't know the details of them, nor  
which is better for your specific purpose, you'll have to read a bit of  
literature).

best regards

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Lorenzo Paulatto
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