[Pw_forum] Question about K-points

[Lorenzo Paulatto]

In data 23 agosto 2009 alle ore 18:46:26, Bipul Rakshit  
<bipulrr at gmail.com> ha scritto:
> I have a question about phonon calculation.
> I am doing phonon of K2S (antifluorite structure), using 10x10x10
> k-points mesh. In that calculations i find -ve frequencies.
>
> But then on increasing the mesh 14x14x14 and also the kinetic energy
> cutoff from 32 to 38 Ry, now i am getting all the +ve freq.

Dear Bipul,
unluckily convergence of physical quantities is not variational with the  
number of k-points; i.e. if you increase the number of k-points you will  
go toward convergence, but not necessarily in a monotone way.

Notwithstanding that, if I had to bet, I would say that the problem was  
the cutoff too low: if the Potassium pseudopotential includes semi-core  
electrons it may be quite hard. Without knowing the detail of your  
calculation there is little more I can honestly say. You could try to  
perform two more calculation: one increasing only the number of k-points  
and one only the cutoff.

best regards

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
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