[Pw_forum] the symmetry problem in calculating phonon frequency of wurtzite AlN

[wz]

Dear all,


I am trying to calculate the phonon frequency of wurtzite AlN in gamma point， i try to use the input files as PHonon/examples/example01/ in Quantum Espresso. The symmetry of wurtzite AlN is C6v, but in the output of my calculation, the symmetry is C3v so that the phonon frequency get by my work do not correspond with that get by reference.  
I want to know that if there is a way in quantum espresso to specify the symmetry? or some other method to get the right symmetry in phonon calculation?
===================================
pw input
 &SYSTEM
    ibrav= 4, celldm(1)= 5.8804, nat= 4, ntyp= 2,
    celldm(3) = 1.600724
    ecutwfc = 160
 /
 &ELECTRONS
    conv_thr =  1.0d-8
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
    Al   26.982  Al.pbesol-n-kjpaw_psl.0.1.UPF
    N    14.007  N.pbesol-n-kjpaw_psl.0.1.UPF
ATOMIC_POSITIONS crystal
    Al  0.333333343         0.666666687         0.000000000
    Al  0.666666627         0.333333313         0.500000000
    N   0.333333343         0.666666687         0.381929994
    N   0.666666627         0.333333313         0.881929994
       
K_POINTS automatic
2 2 1 1 1 1  
------
ph input


phonons of aln at Gamma
 &inputph
  tr2_ph=1.0d-14,
  prefix='aln',
  epsil=.true.,
   outdir='/tmp',
   fildyn='aln.dyn',
 /
0.0 0.0 0.0
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