[Pw_forum] Band gap hybrid functionals

[stefano de gironcoli]

Dear Giuseppe, dear all,

   I think that it should be possible to
   -1) run an scf hybrid calculation with a number of empty bands
   -2) use wannier90 to extract the MLWF (meaning the shortest-ranged TB 
hamiltonian that gives those eigenvalues). Some dependence on the 
disentanglement of the conduction bands would result but probably it 
would not affect low-lying CB states.
   -3) draw the k-dependent band structure of the resulting TB model 
that should reproduce the full calculation in the points that where 
present in the original scf calculation.

   for dense enough grids everything should make sense and converge to 
the right results.
   how fast this happens I don't know but I would be hopeful

   good luck

stefano

On 11/11/2015 14:09, Giuseppe Mattioli wrote:
> 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
>>>
>