[Pw_forum] Band gap hybrid functionals

[Giuseppe Mattioli]

Dear all

> So if I use an 8 atom Si cell, and use fixed
> occupations with 32 bands, the difference between the highest occupied and
> lowest unoccupied is my actual band gap?

This is true for direct band gap semiconductors, where both VBM and  
CBM can be found at Gamma, which must be included in the list. This is  
easy because every N, N, N, 0, 0, 0 automatically generated grid  
includes Gamma. However, in the case of bulk Si the CBM falls  
somewhere along the Gamma-X direction, close to X. If your k-point  
mesh is *very* dense, then the CBM found by the code at a given  
k-point will be negligibly shifted wrt the proper CBM. Otherwise you  
should perform the calculation by using the proper 2-atom Si cell  
(faster calculation with EXX functionals...) and you should be sure to  
include one k-point close to the CBM in the mesh. You can still work  
with the sc 8-atom cell, if you prefer. But you must *fold* the 8-atom  
sc cell into the 2-atom fcc cell.
HTH
Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting Ref Fymz <reffymz at gmail.com>:

> Hey,
>
> Thanks for the reply. So if I use an 8 atom Si cell, and use fixed
> occupations with 32 bands, the difference between the highest occupied and
> lowest unoccupied is my actual band gap? (i.e the one I can compare to PBE
> and experiment)
>
>
> Phil
>
> On 11 November 2015 at 10:13, Ref Fymz <reffymz at gmail.com> wrote:
>
>> Hey,
>>
>> I've tried searching the forum, and I haven't come across a way to
>> calculate the band gap in a material using the hybrid functionals. I know
>> you can't do nscf calculations, but is there any simple way to calculate
>> the band gap for a material using scf simulations?
>>
>> Thanks,
>>
>> Phil
>>


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    Giuseppe Mattioli
    CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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