[Pw_forum] Phonon Dispersion using matdyn.x

[ankit jain]

Dear QE developers and users,

I am using ph.x, q2r.x, and matdyn.x to calculate a phonon dispersion of a
one-dimensional polymer (I am using vacuum in other two directions).
I am using a phonon qgrid of 1*1*6 for the calculation of dispersion from
ph.x. Later using q2r.x and matdyn.x, I am calculation phonon dispersion
for 1*1*20 phonon qgrid.

My problem is that for the phonon qgrid of 1*1*6, I get non-negative
frequencies at all of the irreducible phonon q points, but when I use
matdyn.x to calculate phonon frequencies on 1*1*20, some of my acoustic
modes frequencies are negative for non-gamma points!
For instance, if my irreducible qpoints are 0.0, 0.1,0.3,0.5 (let say),
then I get non-negative frequencies from ph.x for all four of these q
points (which is right), but when I use matdyn.x to intepolate the
frequencies, I get negative frequencies for lowest frequency acoustic
phonons with q-points less than 0.1.

I am using NORMCONS PP, electron k-grid = 1*1*12, with ecutwfc = 100 Ry,
conv_thr=1.0d-12, tr2_ph = 1.0d-14, zasr/asr='one-dim'

I am attaching my pw.in, ph.in, q2r.in and matdyn.in files with this mail.

Thanks,
Ankit Jain
IIT Indore,
India
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