[Pw_forum] Strange phonon branches appear in phonon dispersion

[Viet Bac]

Dear Pw_users,

I am trying to do calculation phonon dispersion and EPC for 1D Aluminum
chain. I got the results which are strange phonon branches appearing. In the
case of 2 atom-cell, I got 4 phonon branches, two of them are Optical and
Acoustical phonon branches and two others, which have zero and negative
values, are unknown for me. In the case if I have 1 atom-cell, that strange
branch also appears. Could any body explain for me what kind of frequencies
here?

I was surfing this forum and FAQ and I found many discussion about negative
frequencies, but I still cannot find the answers for me.

Bac


This is my relaxed unitcell

For 1-atom cell

  input alat =      20.0000 (a.u.)

CELL_PARAMETERS (alat)
   0.999558675   0.000000000   0.000000000
   0.000000000   0.999558675   0.000000000
   0.000000000   0.000000000   0.224680571

ATOMIC_POSITIONS (angstrom)
Al       0.000000000   0.000000000   0.000000000

For 2-atom cell

CELL_PARAMETERS (alat)
   1.000000000   0.000000000   0.000000000
   0.000000000   1.000000000   0.000000000
   0.000000000   0.000000000   0.453300000

ATOMIC_POSITIONS (angstrom)
Al       0.000000000   0.000000000   2.399000000
Al       0.000000000   0.000000000   0.000000000

And INPUT for ph.x

 tr2_ph=1.0d-12,
  prefix='al_20',
  fildvscf='Al2v',
  amass(1)=26.98,
  outdir='/root/tmp/',
  fildyn='al2.dyn',
  elph=.true.,
  trans=.true.,
  ldisp=.true.
  nq1=1, nq2=1, nq3=10

------------------------------
Phung Viet Bac
Division of Mathematical and Physical Science,
Graduate School of Natural Science and Technology,
Kanazawa University,
Kakuma, Kanazawa 920-1192, JAPAN
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