[QE-users] Optical branches are not continuous at gamma point in phonon dispersion

[Changpeng Lin]

Dear all,

I calculated the phonon dispersion of polar materials by DFPT in ph.x and
set  epsil = .true. and trans = .true. to determine the dielectric tensor
and Born effective charges.

I then get the phonon dispersion by matdyn.x and the input set as following,
&input
        flfrc='TE.frc',
        flfrq='TE.frq'
        asr='simple',
        q_in_band_form=.true.
        q_in_cryst_coord=.true.
/
6
6
0.5  0.5 -0.5  100   !Z
0.0  0.0  0.0  100   !G
0.0  0.0  0.5  100   !X
0.25 0.25 0.25 100   !P
0.0  0.5  0.0  100   !N
0.0  0.0  0.0  1     !G

I found that optical branches are not continuous at gamma point. I guess
this is becasue the wrong LO-TO spiliting relation I got. How can I check
it?

More interestingly, I recently tested the phonon dispersion of PbTe by
supercell small displacement method and also determine the dielectric
tensor and Born effective charges by DFPT at gamma point only in ph.x. I
found that there was no LO-TO splitting at gamma point from the output of
dynmat.x. However, if I put the dielectric tensor and Born effective
charges by DFPT into Phonopy and combine the force constants obtained
from small displacement method, I get the correct LO-TO splitting from
Phonopy. So, where is wrong?


Thanks,
Changpeng
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