[Pw_forum] question for phonon calculation

[潘登]

Dear All Users,

I was going to calculating phonon frequency of MgB2 at Gamma point.But I got
the minus value at the lowest three branch.

     q = (    0.000000000   0.000000000   0.000000000 )

 **************************************************************************
     omega( 1) =      -1.333147 [THz] =     -44.469304 [cm-1]
     omega( 2) =      -1.333147 [THz] =     -44.469304 [cm-1]
     omega( 3) =      -1.003834 [THz] =     -33.484515 [cm-1]
     omega( 4) =       9.706117 [THz] =     323.763351 [cm-1]
     omega( 5) =       9.706117 [THz] =     323.763351 [cm-1]
     omega( 6) =      11.666010 [THz] =     389.138778 [cm-1]
     omega( 7) =      15.807885 [THz] =     527.297796 [cm-1]
     omega( 8) =      15.807885 [THz] =     527.297796 [cm-1]
     omega( 9) =      20.623897 [THz] =     687.943739 [cm-1]
 **************************************************************************

I had already relax the lattice.And below is my inputs for scf and phonon
calculation.Any suggest and comments on how to avoid the minus values was
appreciated.

scf.in
 &control
     calculation = 'scf'
     restart_mode='from_scratch'
     prefix='MgB2',
     pseudo_dir = '//Quantum-Espresso/espresso-4.0.3/pseudo/',
     outdir='/tmp/'
 /
 &system
     ibrav=4,
     celldm(1)=5.8398,
     celldm(3)=1.1420,
     nat=3,
     ntyp=2,
     ecutwfc = 45.0,
     ecutrho = 450.0,
     occupations= 'smearing',
     degauss= 0.03,
     smearing= 'cold',
 /
 &electrons
     mixing_beta = 0.7
     conv_thr =  1.0d-12
 /
ATOMIC_SPECIES
 Mg 24.3 Mg.pw91-np-van.UPF
 B  10.8 B.pw91-n-van.UPF
ATOMIC_POSITIONS
Mg    0    0       0
B     0    0.57735 0.571
B     0.5  0.28868 0.571
K_POINTS {automatic}
 32 32 32 0 0 0

ph.in

 MgB2
&inputph
  amass(1)= 24.3,
  amass(2)= 10.8,
  prefix='MgB2'
  tr2_ph=1.0d-25,
  alpha_mix(1)=0.5,
  iverbosity=1,
  outdir='//tmp/',
 /
0 0 0

I also wonder how to apply 'asr' to my calculation.I could not find any
option for ph.x.

Have a nice day!

Deng Pan
Nanjing University
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