[Pw_forum] Dielectric constant calculations for InAs

[Zhiping Xu]

Dear all:

I am using pw.x and ph.x to calculate the dielectric constant of InAs, and here are some problems with the calculations.

(1) After structural relaxation using vc-relax, the stress is now zero (confirmed using scf calculations for the final structure). 

     convergence has been achieved in  10 iterations

     Forces acting on atoms (Ry/au):

     atom    1 type  1   force =     0.00000000    0.00000000    0.00000000
     atom    2 type  2   force =     0.00000000    0.00000000    0.00000000

     Total force =     0.000000     Total SCF correction =     0.000000


     entering subroutine stress ...

          total   stress  (Ry/bohr**3)                   (kbar)     P=    0.00
   0.00000001  -0.00000000  -0.00000000          0.00     -0.00     -0.00
  -0.00000000   0.00000001   0.00000000         -0.00      0.00      0.00
  -0.00000000   0.00000000   0.00000001         -0.00      0.00      0.00


However, the following phonon calculation predicts not only negative phonon frequency at q = 0, but also very strange dielectric constant.

          Dielectric constant in cartesian axis 

          (    -230.720680077      -0.000000000       0.000000000 )
          (      -0.000000000    -230.720680077       0.000000000 )
          (       0.000000000       0.000000000    -230.720680077 )

          Effective charges (d Force / dE) in cartesian axis

           atom      1   In 
      Ex  (        9.55076        0.00000        0.00000 )
      Ey  (        0.00000        9.55076        0.00000 )
      Ez  (        0.00000        0.00000        9.55076 )
           atom      2   As 
      Ex  (       -9.43886        0.00000        0.00000 )
      Ey  (       -0.00000       -9.43886       -0.00000 )
      Ez  (        0.00000        0.00000       -9.43886 )

     Diagonalizing the dynamical matrix

     q = (    0.000000000   0.000000000   0.000000000 ) 

 **************************************************************************
     freq (    1) =      -6.064621 [THz] =    -202.293974 [cm-1]
     freq (    2) =      -6.064621 [THz] =    -202.293974 [cm-1]
     freq (    3) =      -6.064621 [THz] =    -202.293974 [cm-1]
     freq (    4) =       0.164141 [THz] =       5.475159 [cm-1]
     freq (    5) =       0.164141 [THz] =       5.475159 [cm-1]
     freq (    6) =       0.164141 [THz] =       5.475159 [cm-1]
 **************************************************************************

The input files are attached below for your reference. 

(2) One more question, the vc-relax calculation converges at zero stress, which was calculated using the cut-off set for initial cell parameters, so one has to redo the calculations using the new cell parameters following the suggestion at http://www.quantum-espresso.org/faq/self-consistency/#6.11 <http://www.quantum-espresso.org/faq/self-consistency/#6.11>. Is there any better (more automatic) way to obtain a final relaxed cell/atom structures?

Could any one help me on these two questions?

Thank you,

Zhiping

input files:

—scf.in—
&control
  calculation     = 'scf',
  restart_mode    = 'from_scratch',
  pseudo_dir      = '/home/xuzp/bin/lib/upf_files',
  outdir          = '.',
  prefix          = 'InAs',
  tstress         = .true.
  tprnfor         = .true.
  wf_collect      = .true.
/
&system
  ibrav           = 2, 
  celldm(1)       = 11.69675,
  nat             = 2, 
  ntyp            = 2,
  ecutwfc         = 90,	
/
&electrons
  diagonalization = 'cg'
  conv_thr        =  1.0d-10
/

ATOMIC_SPECIES
 In  111.818  In.pbe-dn-rrkjus_psl.0.2.2.UPF
 As   74.920  As.pbe-n-rrkjus_psl.0.2.UPF

ATOMIC_POSITIONS crystal
 In 0.00 0.00 0.00
 As 0.25 0.25 0.25

K_POINTS automatic 
 9 9 9 0 0 0 

—ph.in—
phonons of InAs
&inputph
  tr2_ph   = 1.0d-16,
  niter_ph = 100,
  prefix   = 'InAs',
  epsil    = .true.		  
  amass(1) = 111.818,
  amass(2) = 74.920,
  outdir   = '.',
  fildyn   = 'InAs.matdyn',
/
0.0 0.0 0.0
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