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[<trp6@case.edu>]

Dear all, 

I am trying the spontaneous polarization of the III-V nitrides. Here is my
input file for the GaN
# Specific to calculation of Berry Phase.
  berryopt -1
  rfdir 1 1 1
#Definition of the planewave basis set
  ecut    100.0
  ectusm  0.5
  dilatmx 1.05
  diemac  6.0

#Specific to ground state calculation
  iscf   5
  nstep  60

#Definition of the unit cell
acell  6.026 6.026  9.798276
angdeg 90 90 120

#Definition of the atom types
ntypat 2
znucl 31 7

#Definition of the atoms
natom 4
typat 1 1 2 2
xred  0.0  0.0  0.0
      2/3  1/3  1/2
      0    0    0.377
      2/3 1/3   0.877

#Gives the number of band, explicitely (do not take the default)
nband  18               # For an insulator (if described correctly as an
insulator

#Exchange-correlation functional
ixc 1

#Definition of the k-point grid
ngkpt 4 4 8
nshiftk 1
shiftk 0 0 .5
kptopt    3
toldfe    1.0d-12
nbdbuf 0

I used hgh pseudopotential, hence used high cutoff. But my result for the zz
part of polarization is as follow 
 Computing the polarization (Berry phase) for reciprocal vector:
  0.00000  0.00000  0.12500 (in reduced coordinates)
  0.00000  0.00000  0.01276 (in cartesian coordinates - atomic units)
 Number of strings:    16
 Number of k points in string:    8

 Summary of the results
 Electronic Berry phase     9.622896788E-01
            Ionic phase     7.700000000E-01
            Total phase     1.732289679E+00
    Remapping in [-1,1]    -2.677103212E-01

           Polarization    -8.512874458E-03 (a.u. of charge)/bohr^2
           Polarization    -4.870620906E-01 C/m^2


 Polarization in cartesian coordinates (a.u.):
 (the sum of the electronic and ionic Berry phase has been fold into [-1, 1])
     Electronic:  0.399941195E-12  -0.113753605E-11   0.305996840E-01
     Ionic:       0.318961478E-16  -0.868508257E-17   0.244850976E-01
     Total:       0.399969197E-12  -0.113755148E-11  -0.851287446E-02

 Polarization in cartesian coordinates (C/m^2):
 (the sum of the electronic and ionic Berry phase has been fold into [-1, 1])
     Electronic:  0.228825405E-10  -0.650838549E-10   0.175075365E+01
     Ionic:       0.182493052E-14  -0.496914937E-15   0.140090904E+01
     Total:       0.228841426E-10  -0.650847378E-10  -0.487062091E+00

But the total zz polarization of-0.48 is more then a order of magnitude 
greater
then reported p_zz of -0.029 reported in Phys. Rev B 56, R10024. I don't
understand the differences as both of them are similar first principle
calculation.   So I am wondering, if I am wrong ?. I played with the fhi
psedudopotential and obvious tests with the cutoff kpoints , Berryopt, but the
result is same. 

I would appreciate if anyone help me how to calculate the spontaneous
polarization correctly.  

Thanks in an advance.
 
Tularam