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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