[Pw_forum] the negative phonon mode of ZnS

[Sun ShaoRui]

Dear all:
I have calculated the phonon of ZnS with zincblende structure under anbient 
condition, and I get the pseudopotentials from the pwscf web page. For the 
gamma point, the result seems right, but fro the X point, there are some 
negative frequency modes in the output file. 
The pw input file is shown as follows:
-----------------------------------------------------------------------------
----------------
&CONTROL
  calculation = 'phonon',
  restart_mode = 'from_scratch',
  ndr = 51,
  ndw = 51,
  nstep  = 500,
  iprint = 10,
  tprnfor = .TRUE.,
  dt    = 5.0d0,
  etot_conv_thr = 1.d-9,
  forc_conv_thr = 1.d-4,
  pseudo_dir = '/home/wien2k/pw/pseudo',
  outdir = '/home/wien2k/pwwork/ZnS'
  prefix = 'ZnS-X'
/

&SYSTEM
  ibrav = 2, 
  celldm(1) = 10.231926,  
  nat  = 2,
  ntyp = 2,
  ecutwfc = 20.0,
  smearing = 'f-d'
  nspin = 1,
/

&ELECTRONS
  mixing_beta = 0.7,
  conv_thr = 1.0d-8,
/
&phonon
    xqq(1)=1.0, xqq(2)=0.0, xqq(3)=0.0
 /
 ATOMIC_SPECIES
  S  32.0d0  S.pbe-van_bm.UPF   
 Zn  65.4d0  Zn.pbe-van.UPF      

ATOMIC_POSITIONS (alat)
   Zn    0.0000    0.0000    0.0000  1 1 1
   S     0.2500    0.2500    0.2500  1 1 1
K_POINTS automatic 
  8 8 8   1 1 1
******************************************

And the ph input file is shown as follows:
*******************************************
phonons of ZnS at X
 &inputph
  tr2_ph=1.0d-14,
  prefix='ZnS-X',
  amass(1)=65.39,
  amass(2)=32.066,
  outdir='/home/wien2k/pwwork/ZnS/',
  fildyn='ZnS-X.dyn',
 /
 1.0   0.0   0.0
*********************************************************
The phonon mode list in the ph output program is shown as follows:
  **************************************************************************
     omega( 1) =     -32.553715 [THz] =   -1085.882263 [cm-1]
     omega( 2) =     -32.447538 [THz] =   -1082.340572 [cm-1]
     omega( 3) =     -32.447538 [THz] =   -1082.340572 [cm-1]
     omega( 4) =      10.118904 [THz] =     337.532556 [cm-1]
     omega( 5) =      10.118904 [THz] =     337.532556 [cm-1]
     omega( 6) =      10.391189 [THz] =     346.615057 [cm-1]
 **************************************************************************
I am puzzled about the three negative frequency modes for the stable state. 
I also tried to use other kinds of pseudopotentials which are produced by 
the fhi98PP and uspp, respectively, and increase the energy cut-off to 60 
Ry.. But all the results are similar.
 I think for the stable crystal system, all the phonon mode should not be 
negative. From the former discussion, somebody have talked about the reason 
about the negative mode, and I have tried to consider that in my computation.
 Who could give me some suggestions and comments?
Best regards!
                                                     Shaorui Sun