[Pw_forum] phonon frenquecies convergences

[Stefano Baroni]

On Jan 9, 2012, at 12:17 PM, Paolo Giannozzi wrote:

> On Mon, 2012-01-09 at 10:11 +0000, hichem bouderba wrote:
> 
>> Suppose that one performs a convergence study for just one q-point
>> (say Gamma for example), is it reasonable to expect that results will
>> be reliable for other q-points ?
> 
> if you check convergence wrt cutoff, or wrt k-points in insulators, 
> I would say it is reliable. Convergence wrt k-points may be very 
> tricky in metals, though, due to the presence of Kohn anomalies.

One more word of caution. You should understand what convergence stems from. If you think about it, you will realize that phonon frequencies depend critically on electronic screening. The latter, in turn, is expressed in terms of the independent-electron susceptibility, whose expression contains differences between empty and conduction energies at the denominator. That's the technical reason for Kohn anomalies. I do not have examples handy, but I could imagine cases where E_c(k+q)-E_v(k) may depend sensitively on q (typically, in a semi-metal), for which the convergence of phonon frequencies on q may be not so obvious. Again, I think that Paolo's answer is correct as a rule of thumb, but you better make sure you understand why ...

SB


---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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