[Pw_forum] Dielectric constant with Lyddane-Sachs-Teller relation

[Paolo Giannozzi]

TuanAnh Pham wrote:

 > Could you please let me know what is the different between these two 
methods ?

they should give the same results, when both methods apply

 > Is that correct that the LST relation only works for the system with 
sufficient symmetry?

typically the LST relation is derived for high-symmetry crystals
with two atoms in the unit cell, in which all you have is one TO
and one LO nonzero frequency at Gamma. The LST relation can be
generalized, though. See for instance this old paper:
W. Cochran and R. A. Cowley, J. Phys. Chem. Solids 23, 447 (1962)

Paolo
-- 
Paolo Giannozzi, Democritos and University of Udine, Italy