[Pw_forum] Band gap hybrid functionals

[Giuseppe Mattioli]

Dear Phil

> But I mentioned a problem with using a very dense k - point grid and an 8
> atom Si cell, the program exits before it can output the forces, stresses,
> and the individual contributions to the total energy

Sorry, but this was not mentioned in the current thread in which you  
asked how to calculate the band gap...

> Nobody was able to point me in the right direction with that. Also, another
> user has previously reported that using a 2 atom si cell results in an
> oscillation in the total energy as you increase ecutwfc (something I have
> also verified)

Have you checked if the Gamma gap or the "best" gap (at fixed  
occupations) jumps like the total energy or if it is already converged?

However, both Stefano's and Mostafa's suggestions deserve to be considered.

Have a nice computation and don't forget to add your affiliation to  
the posts to this forum.

Giuseppe

Giuseppe Mattioli
ISM-CNR
Italy

Quoting Ref Fymz <reffymz at gmail.com>:

> But I mentioned a problem with using a very dense k - point grid and an 8
> atom Si cell, the program exits before it can output the forces, stresses,
> and the individual contributions to the total energy (hence why regardless
> of what screening parameter I used I got the same band gap, as the fock).
> Nobody was able to point me in the right direction with that. Also, another
> user has previously reported that using a 2 atom si cell results in an
> oscillation in the total energy as you increase ecutwfc (something I have
> also verified), so I'm not sure I can accurately calculate the band gap
> using a 2 atom cell either.
>
> Phil
>
> On 11 November 2015 at 12:18, Ref Fymz <reffymz at gmail.com> wrote:
>
>> 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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