[QE-users] phonon frequencies at Gamma using qplot = .true. along K-Gamma-L

[Hari Paudyal]

Dear experts,

To calculate the phonon along a specific path of BZ (K-Gamma-L) for Si, I
used   qplot = .true. and q_in_band_form = .true.  (taking 2 points in
between).  Surprisingly, I got the following frequencies at Gamma,
     q = (    0.000000000   0.000000000   0.000000000 )

 **************************************************************************
     freq (    1) =      11.535533 [THz] =     384.783979 [cm-1]
     freq (    2) =      11.535533 [THz] =     384.783979 [cm-1]
     freq (    3) =      11.535533 [THz] =     384.783979 [cm-1]
     freq (    4) =      18.554595 [THz] =     618.914662 [cm-1]
     freq (    5) =      18.554595 [THz] =     618.914662 [cm-1]
     freq (    6) =      18.554595 [THz] =     618.914662 [cm-1]
 **************************************************************************

On the other hand, If I calculate only at Gamma point, I get
     q = (    0.000000000   0.000000000   0.000000000 )

 **************************************************************************
     freq (    1) =      -0.117104 [THz] =      -3.906180 [cm-1]
     freq (    2) =      -0.117104 [THz] =      -3.906180 [cm-1]
     freq (    3) =      -0.117104 [THz] =      -3.906180 [cm-1]
     freq (    4) =      15.035771 [THz] =     501.539319 [cm-1]
     freq (    5) =      15.035771 [THz] =     501.539319 [cm-1]
     freq (    6) =      15.035771 [THz] =     501.539319 [cm-1]
 **************************************************************************

Why am I getting the large frequencies at Gamma if I calculate along
K-Gamma-L? Do I need to specify Gamma always in the first?Why?

My input files for the scf and phonon for Si are as the following.

 &control
    calculation     = 'scf'
    prefix          = 'Si'
    restart_mode    = 'from_scratch'
    wf_collect      = .true.
    pseudo_dir      = './'
    outdir          = './'
 /
 &system
    ibrav           = 2
    celldm(1)       = 10.261217
    nat             = 2
    ntyp            = 1
    ecutwfc         = 45.0
 /
 &electrons
    diagonalization = 'david'
    mixing_beta     = 0.3
    conv_thr        = 1.0d-10
/
ATOMIC_SPECIES
  Si  28.0855  Si.pz-vbc.UPF
ATOMIC_POSITIONS alat
  Si  0.00  0.00  0.00
  Si  0.25  0.25  0.25
K_POINTS automatic
8 8 8 0 0 0

Si
&inputph
 outdir    = './',
 prefix    = 'Si',
 fildyn    = 'Si.dyn',
 fildvscf  = 'dvscf',
 ldisp     = .true.,
 tr2_ph    = 1.0d-14
 qplot     = .true.
 q_in_band_form = .true.
/
3
-0.7500000   0.7500000   0.0000000   2   ! K
 0.0000000   0.0000000   0.0000000   2   ! G
-0.5000000   0.5000000   0.5000000   1   ! L

Thank You,
Hari Paudyal
SUNY Binghamton
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