> 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 d?partement de physique Universit? de Batna Alg?rie
