[Pw_forum] Problem with nbnd in clusters

[Cyrille Barreteau]

Stefano de Gironcoli wrote:

>Nicola Marzari wrote:
>  
>
>>6) this open up my favourite question - should B be spherical or
>>cylindrical in DFT ? I.e. should we look like for occupation 1 0 0
>>for the 2p levels, or 1/3 1/3 1/3 ?
>>  
>>7) Last, but important - even using Gamma sampling, PWSCF finds
>>for your geometry 16 symmetry operations, and thus heavily
>>symmetrizes the system. That means that in 6) even 1 0 0 would
>>become spherical. I.e. the symmetrization has a similar effect
>>to temperature, in creating a charge density that is more symmetrical 
>>/degenerate occupations than it could be.
>>
>>I would be *Very curious* to see what happens if you were to
>>put the dimer in a very random position, so the code misses most
>>of the symmetry operations, and also you specify no_sym, so no 
>>symmetrization is performed.
>>			nicola\
>>
>>  
>>    
>>
> I agree with Nicola that this is very relevant and that symmetrization
> induces
> a degeneracy that could not be there in the best "broken symmetry" 
> solution.
> This one can be achieved by 1) occupations='from_input' 2) nosym=.true.
> no smearing is required then.
>
I agree that symmetrization (due to the periodization) induces a 
degeneracy but the
"real" system that I want to simulate (a dimer alone in space) has even more
symmetries. In particular it has a rotational symmetry that will never 
be there
in a periodic code!! life is hard:-)
Anyway I will try to put the dimer in a random position to see what happens.

Systems with discrete levels are always a bit tricky and often present
multiple solutions..

  cyrille