Dear QE users and developers I'm trying to understand the shape of the normal modes in the diamond structure and to calculate the frequency in the X symmetry point using supercell (in addition to the regular DFPT calculation of QE). The atoms in the unit cell in diamond structure are in (0,0,0) and (0.25,0.25,0.25). The wave-vector in X is (2 pi/a,0,0). Therefore, these two atoms have a phase difference of pi/2. Because of that I do not understand how to calculate the frequency using supercell because there are no times in which the kinetic energy are zero. Do I need to calculate the kinetic energy in these kind of modes? can I do it classically? Thank you in advance Uri Argaman Ben-Gurion University Material Engineering Department Israel
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