[Pw_forum] The negative frequencies in the calculation of phonon.

[shoutian sun]

>Dear all,
>I have a problem in calculating phonons of a material.I think the ecutwfc = 44.0982(600eV) 
>and the K_POINTS is enough(in vc-relax I set it 10 10 8). The forces of atoms is also list 
>below. But the frequencies in *.dyn1 is negative(about -1.67895THz). 
&CONTROL
   calculation =   'scf' ,
   restart_mode = 'from_scratch' ,
   outdir='/home/user/ecnuphyk/SUN/tmp/tmp-p4/' ,
   pseudo_dir = '/home/user/ecnuphyk/SUN/pseudo/' ,
   prefix='...' ,
   tstress = .true. ,
   tprnfor = .true. ,
   etot_conv_thr = 1.0D-5 ,
   forc_conv_thr = 1.0D-4 ,
 /
 &SYSTEM
   ibrav = 0 ,
   celldm(1) = ...  ,
                         nat  =  16 ,
                        ntyp  =  2 ,
                       nspin  =  1 ,
                     ecutwfc  = 44.0982 ,#about 600eV
 /
 &ELECTRONS
                    conv_thr  =  1.0d-7  ,
                  mixing_beta = 0.7 ,
 /
CELL_PARAMETERS (alat)
...
ATOMIC_SPECIES
...
ATOMIC_POSITIONS (crystal)
...
K_POINTS automatic
 8 8 6 0 0 0
*************The forces of the atoms in the self-consistent calculation:
     atom   1 type  1   force =     0.00000085   -0.00000187   -0.00000180
     atom   2 type  1   force =    -0.00000200    0.00000092    0.00000220
     atom   3 type  1   force =     0.00000200   -0.00000092   -0.00000220
     atom   4 type  1   force =    -0.00000085    0.00000187    0.00000180
     atom   5 type  2   force =    -0.00002835    0.00003010   -0.00000078
     atom   6 type  2   force =    -0.00002239   -0.00001414    0.00000083
     atom   7 type  2   force =     0.00001800    0.00001850    0.00000131
     atom   8 type  2   force =     0.00000012   -0.00000010   -0.00000065
     atom   9 type  2   force =     0.00002757   -0.00002933   -0.00000124
     atom  10 type  2   force =    -0.00000019   -0.00000006    0.00000066
     atom  11 type  2   force =     0.00000019    0.00000006   -0.00000066
     atom  12 type  2   force =    -0.00002757    0.00002933    0.00000124
     atom  13 type  2   force =    -0.00000012    0.00000010    0.00000065
     atom  14 type  2   force =    -0.00001800   -0.00001850   -0.00000131
     atom  15 type  2   force =     0.00002239    0.00001414   -0.00000083
     atom  16 type  2   force =     0.00002835   -0.00003010    0.00000078
********************ph.in
phonons of ...
&inputph
  tr2_ph=1.0d-12,
  prefix='...',
  ldisp=.true.,
  nq1=4, nq2=4, nq3=3
  epsil=.true. ,
  ...
  outdir= '/home/user/ecnuphyk/SUN/tmp/tmp-p4/' ,
  fildyn='....dyn',
 /
>*********************************


       
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