[Pw_forum] Problem with nbnd in clusters

[Matteo Cococcioni]

Dear Cyrille,

Nicola Marzari's is a good advice for these systems. Another thing that 
you could check is starting
from random wave functions (instead of atomic). I am not expert of 
diagonalization algorithms
but maybe what happens is that davidson with 20 bands (who knows why...) 
gets trapped in some local minimum
just for numerical reasons. So maybe some noise could help.
How do the energies compare?

Regards,

Matteo


Nicola Marzari wrote:
> Hi Cyrille,
>
> your tm dimer will likely require spin polarization, and
> will have several scf solutions that correspond to different
> S_z (that you can fix in several ways, either specifying the
> number of spin up and spin down, or using a fermi_up and
> fermi_down energy, to specify n_up - n_down but still treating
> the system with a smearing and fractional occupations).
>
> Even for a given S_z, there will be different scf solutions
> that have different spatial symmetry.
>
> So, in your case I would definitely switch spin-polarization,
> and maybe play around with small and large smearings, to see what
> happens, and look at the states, and the occupations. Anyhow, if
> you get to a scf solution, that is a "good" solution for the GGA
> problem, although might not be very physical.
>
> Also, have a look at Kulik et al PRL 2006.
> 	
> 			nicola
>
>
> Cyrille Barreteau wrote:
>   
>> Dear pwscf_ers
>>
>> I am doing a very simple calculation on a transition metal
>> dimer and I have a encoutered a strange problem (I love
>> strange problems:-)
>>
>> I have done a first calculation with the default value
>> of nbnd, ie nbnd=nelec/2*20% (=12 in my case)
>>
>> But since it is known that in clusters it is often good
>> to increase nbnd I have performed another calculation with
>> larger nbnd (=20). The result is quite different from
>> the one at nbnd=12.
>>
>> I have then increased again nbnd up to 25 and then I recover
>> the result obtained for nbnd=12
>>
>> More problematic is the local density of states (I know
>> it is not really a dos but a bunch of dirac peaks).
>> In a dimer with the z axis along the direction connecting
>> the two atoms, the xy and x^2-y^2 dos should be degenerate
>> (if the supercell box is large enough).
>> In fact the xy and x^2-y^2 are degenerate if nbnd=12 or 25
>> but there is a rather large splitting if nbnd=20.
>>
>> I am quite sure the result for nbnd=20 is not correct but
>> what is the origin of this problem?
>>
>> Maybe I could try to use another diagonalization scheme..
>>
>>    thanks for reading my strange problems
>>
>>      cyrille
>>
>>
>>     
>
>
>   

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