[Pw_forum] Hello, why the calculated polarization through Berry Phase is so large

[Xiaofang]

Dear all,
  I have calculated the polarization of KNbO3 using the Berry Phase. The atomic position and lattice constants of input are all same with the experimental values. The computed polarization is 1.1201929 C/m^2,but the experimental polarization is about 0.37 C/m^2. I even use the espressov.4.0cvs to calculate, but the result is similar. I have also tried to calculate the Born effective charge using ph.x, through the effective charge I get the polarization which is 0.44 C/m^2. I don’t know why there is so much difference between the two kinds of method and why the calculated polarization through Berry phase is so large. Any suggestion is appreciated.
  This is the Scf’s input
  &control
      calculation  = 'scf'
      restart_mode = 'from_scratch'
      pseudo_dir   = '/home/pseudo/'
      outdir       = '/home/tmp/'
   /
   &system
      ibrav=6
      celldm(1)=7.5532353215993782928038366689241
      celldm(3)=1.016512384288216162121591193395
      nat=5
      ntyp=3
      nbnd=20
      ecutwfc=30.0
      ecutrho =300.0
      occupations = 'fixed'
      degauss=0.00
   /
   &electrons
      diagonalization ='cg'
      conv_thr = 1e-12,
      mixing_beta=0.3,
   /
  ATOMIC_SPECIES
    K    39.0983   019-K-ca-sp-vgrp.uspp.UPF 
   Nb   92.90638   041-Nb-ca-sp-vgrp.uspp.UPF 
   O    15.9994    008-O-ca--vgrp.uspp.UPF
  ATOMIC_POSITIONS (crystal)
  K        0.5           0.5           0.518
  Nb       0.000000000   0.000000000   0.0
  O        0.500000000   0.00000000    0.04
  O        0.00000000    0.500000000   0.04
  O        0.00000000    0.00000000    0.544
   
  K_POINTS {automatic}
     6 6 3 1 1 1
   
  This is the BP’s input
  &control
      calculation   = 'nscf'
      pseudo_dir    = '/home/pseudo/'
      outdir        = '/home/tmp/'
      lberry        = .true.
      gdir          = 3
      nppstr        = 9
   /
  &system
      ibrav=6
      celldm(1)=7.5532353215993782928038366689241
      celldm(3)=1.016512384288216162121591193395
      nat=5
      ntyp=3
      nbnd=20
      ecutwfc=30.0
      ecutrho =300.0
      occupations = 'fixed'
      degauss=0.00     
   /
   &electrons
      diagonalization ='cg'
      conv_thr      = 1e-5
      mixing_beta   = 0.3
   /
  ATOMIC_SPECIES
    K    39.0983   019-K-ca-sp-vgrp.uspp.UPF
   Nb   92.90638   041-Nb-ca-sp-vgrp.uspp.UPF 
   O    15.9994    008-O-ca--vgrp. uspp.UPF
  ATOMIC_POSITIONS (crystal)
  K        0.5           0.5           0.518
  Nb       0.000000000   0.000000000   0.0
  O        0.500000000   0.00000000    0.04
  O        0.00000000    0.500000000   0.04
  O        0.00000000    0.00000000    0.544
   
  K_POINTS {automatic}
    6 6 9  1 1 1
   
  Thanks in advance.
  Zhanfang Li
  Joint training student, Penn State University, USA

       
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