[Pw_forum] specify occupation with k-points

[Guido Roma]

Paolo Giannozzi wrote:
> Stefano Baroni wrote:
>
>   
>> On top of that, I would say that there would be little point to use
>> k-point sampling to describe a defect, which would require a supercell
>> in any case.
>>     
>
> yes and no. There are cases in which relatively small supercells
> describe well the defect (i.e. the interaction of the defect with its
> periodic replica is small) but do not describe well the underlying
> crystal structure if one uses the Gama point only. In those cases,
> using k-points makes sense.
>
> Paolo
>   
There are also cases (for example Si interstitials in SiC) in which a 
defect level in the gap is still dispersed with relatively large 
supercells but using K-points allows to have relatively well converged 
total (i.e. formation energy). It has been proposed (Bernardini et al. 
Eur. Phys. J. B, v38, p437 (2004)) to impose the same occupation 
occuring at Gamma to the other k-points (where spurious breaking of 
degeneracies could appear). My impression is that this cumbersome 
procedure can be dangerous as, I believe, it does not give the DFT 
ground state (which, as badly converged as it can be with the supercell 
size, is what we are looking for).
I would be happy to have the feeling of others in the list on this point.

Guido
-------------- next part --------------
A non-text attachment was scrubbed...
Name: guido_roma.vcf
Type: text/x-vcard
Size: 260 bytes
Desc: not available
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20090206/f9466b5e/attachment.vcf>