[Pw_forum] negative dielectric constant

[Hua Bao]

Dear PWusers,

I am trying to perform phonon calculation to thin layer silicon structure.
When I do the single layer structure, the results look fine.

Now I follow the same procedure and performed phonon calculation for double
layer silicon. The structure is a literature reported stable structure and
have no negative frequencies. I also performed vc-relax before the scf
calculation. However, I find that the dielectric constant is negative. See
below.

      thresh= 0.345E-09 alpha_mix =  0.700 |ddv_scf|^2 =  0.924E-18

     End of electric fields calculation

          Dielectric constant in cartesian axis

          (     519.702858197    -231.772865836       0.075753747 )
          (    -231.772894836     802.730592803      -0.157103615 )
          (       0.075591420      -0.156819902       1.555242942 )


I am wondering what could be the possible reason for this.
Below is my SCF file. The phonon file and vc-relax files are also attached.

 &control
    calculation='scf'
    restart_mode='from_scratch',
    !pseudo_dir='directory where pseudopotentials are stored/',
    !outdir='directory where large files are written/'
    pseudo_dir='./',
    outdir='./tmp'
    prefix='PH',
 /
 &system
    ibrav=4, celldm(1)=7.18944, celldm(3)=5, nat=4, ntyp=1,
    ecutwfc =60.0
 /
 &electrons
    conv_thr =  1.0d-8
    mixing_beta = 0.7
/
&IONS
  bfgs_ndim         = 1,
  !upscale           = 100.D0,
  pot_extrapolation = "second_order",
  wfc_extrapolation = "second_order",
 /
ATOMIC_SPECIES
 Si  28.0855 Si.pz-vbc.UPF
ATOMIC_POSITIONS
Si      0       0       0.049791982
Si      0       0.569680156     0.217896315
Si      0       0.569376936     0.864095041
Si      0.493360901     0.284531641     1.032253949
K_POINTS automatic
  32 32 1 0 0 0
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