[Pw_forum] phonon frenquecies convergences

[hichem bouderba]

> 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

thank you for the valuable explanation !  I will take the time for
thinking about it !

-- 
Mr. Hichem Bouderba
département de physique
Université de Batna
Algérie