[Pw_forum] Phonon dispersion relation K shifted

[Luigi Bagolini]

Dear Users,
I am trying to do the phonon dispersion relation for crystalline Silicon
along the lines Gamma,K,X,Gamma,L,X,W,L.
I'm encountaring the following problem: the shape of the dispersion curves
seems very good but there is an evident shift
with respect ot the K points. In particular the shift icreases while moving
form left to right (Gamma to L).
Such shift is evident in the plot I include in the present email.

I use the standard procedure to generate phonon dispersion relations.

1) pw.x calculation:
    &control
     calculation='scf'
     restart_mode='from_scratch',
     pseudo_dir = './',
     outdir='./cSi',
     prefix='cSi'
   /
   &system
    ibrav=2, celldm(1)=10.20, nat=2, ntyp=1,
    ecutwfc =16.0
   /
   &electrons
    conv_thr =  1.0d-8
    mixing_beta = 0.7
   /
   &ions
   /
  ATOMIC_SPECIES
  Si  28.0855 Si.pz-vbc.UPF
  ATOMIC_POSITIONS
  Si 0.00 0.00 0.00
  Si 0.25 0.25 0.25
  K_POINTS automatic
  8 8 8 0 0 0

2) ph.x calculation:
   &inputph
    tr2_ph=1.0d-14,
    prefix='cSi',
    ldisp=.true.
    amass(1)=28.0855,
    outdir='./cSi',
    fildyn='dyn',
    nq1=8,
    nq2=8,
    nq3=8,
   /

3) q2r.x calculation:
   &input
    fildyn='dyn',
    zasr='simple',
    flfrc='si.888.fc'
   /

4) matdyn.x :
   &input
    asr='simple',
    amass(1)=28.0855,
    flfrc='si.888.fc'
    flfrq='si.freq'
    q_in_band_form=.true.,
   /
   8
   0.000 0.000 0.000 40
   0.750 0.750 0.000 10
   1.000 0.000 0.000 40
   0.000 0.000 0.000 40
   0.500 0.500 0.500 20
   1.000 0.000 0.000 15
   1.000 0.500 0.000 30
   0.500 0.500 0.500  1

 After this, the file si.freq.gp shows the shift and also using the routine
PlotPhon doesn't help.
 I have cheked the forum for several days and I haven't found anything like
this so I'm sure I am
 doing something wrong, I hope I can have some usefull hints.

 Thanks very much in advance.

Luigi Bagolini
Department of Physics
University of Cagliari, Italy.
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