[QE-users] Negative modes in phonon dispersion calculation

[LEKSHMI S M .]

Hi,
I have been doing the phonon calculation for YbSb2 using PBE GGA
pseudopotential. The structure has been optimized using relax calculation
under energy convergence threshold of 2.4d-07 eV. There are large negative
frequencies appearing in the dispersion.  Could it be due to some
convergence issue or a problem with the pseudopotential? I'm enclosing the
input files for your reference.

*scf.in <http://scf.in>*
&CONTROL
  calculation = 'scf'
  outdir = './'
  prefix = 'ybsb2'
  pseudo_dir = 'pseudo'
  tprnfor = .true.
  tstress = .true.
  verbosity = 'high'
/
&SYSTEM
  ecutrho =   320
  ecutwfc =   80
  ibrav = 0
  nat = 12
  !nosym = .false.
  ntyp = 2
/
&ELECTRONS
  conv_thr =   2.4000000000d-8
  electron_maxstep = 80
  mixing_beta =   7.0000000000d-01
/
ATOMIC_SPECIES
Sb     121.76 Sb.pbe-n-kjpaw_psl.1.0.0.UPF
Yb     173.054 Yb.pbe-spn-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
Yb            0.0000000000        0.1024081695        0.7499999784
Yb            0.0000000000        0.8975918305        0.2500000216
Yb            0.5000000000        0.6024111306        0.7500002082
Yb            0.5000000000        0.3975888694        0.2499997918
Sb            0.0000000000        0.2556987014        0.2499999876
Sb            0.0000000000        0.7443012986        0.7500000124
Sb            0.5000000000        0.7557038701        0.2500000893
Sb            0.5000000000        0.2442961299        0.7499999107
Sb            0.0000000000        0.4380022978        0.7499998700
Sb            0.0000000000        0.5619977022        0.2500001300
Sb            0.5000000000        0.9380020341        0.7500000119
Sb            0.5000000000        0.0619979659        0.2499999881
K_POINTS automatic
9 3 9 0 0 0
CELL_PARAMETERS angstrom
      4.5290000000       0.0000000000       0.0000000000
      0.0000000000       16.6614000000       0.0000000000
      0.0000000000       0.0000000000         4.3040000000

 *ph.in <http://ph.in>*
&INPUTPH
  prefix = 'ybsb2'
  tr2_ph = 1.0d-9
  ldisp = .true.
  nq1 = 2
  nq2 = 2
  nq3 = 2
  fildyn = 'ybsb2.dyn'
  fildvscf = 'dvscf'
/

*q2r.in <http://q2r.in>*
 &input
  zasr='simple',
  fildyn='ybsb2.dyn',
  flfrc='ybsb2.fc'
 /

*matdyn.in <http://matdyn.in>*
&INPUT
  asr = 'simple'
  flfrc = 'ybsb2.fc'
  flfrq = 'ybsb2.freq'
  flvec = 'ybsb2.modes'
  q_in_band_form = .true.
  q_in_cryst_coord = .true.
/

4
0.0000000000 0.0000000000 0.0000000000 20 !G
0.0000000000 0.5000000000 0.0000000000 20 !S
0.0000000000 0.5000000000 0.5000000000 20 !R
0.0000000000 0.0000000000 0.5000000000 20 !Z


Any help on this would be much appreciated. Thanks in advance.

Regards,
Lekshmi SM
Research scholar
BITS Pilani Hyderabad Campus

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