[QE-users] Non-zero polarization in cubic KNbO3

[Rajender Prasad Tiwari]

Dear All,

I want to calculate berry phase polarization in KNbO3(KNO).  Ideally, for a
perfect cubic KNO the polarization should be zero. However, I’m getting
0.4898671 C/m^2. I’ve no idea why am I getting this result. Can anyone
help??
below are my scf.in, nscf.in and polarization part of nscf.out

scf.in

&CONTROL
    calculation = 'scf'
    restart_mode = 'from_scratch'
    outdir = 'c-kno' ,
    pseudo_dir = '/berry-phase-kno'
    prefix = 'bp-KNO'
/

&SYSTEM
    ibrav=1
    celldm(1)=7.59934d0
    nat=5
    ntyp=3
    nbnd=40
    ecutwfc=70
    occupations = 'fixed'
    degauss=0.00

/

&ELECTRONS
    conv_thr    = 1.D-6,
    mixing_beta = 0.07
/

ATOMIC_SPECIES
    K 39.09830 K.pz-hgh.UPF
    Nb 92.90638 Nb.pz-hgh.UPF
    O 15.99940 O.pz-hgh.UPF

ATOMIC_POSITIONS {crystal}
K        0.000000000   0.000000000   0.000000000
Nb      0.500000000   0.500000000   0.500000000
O        0.500000000   0.500000000   0.000000000
O        0.500000000   0.000000000   0.500000000
O        0.000000000   0.500000000   0.500000000

K_POINTS {automatic}
4 4 4  0 0 0





nscf.in

&CONTROL
    calculation = 'nscf'
    restart_mode = 'from_scratch'
    outdir = 'c-kno'
    pseudo_dir = '/berry-phase-kno'
    prefix = 'bp-KNO'
    lberry = .true.
    gdir = 3
    nppstr = 7
/

&SYSTEM
    ibrav=1
    celldm(1)=7.59934d0
    nat=5
    ntyp=3
    nbnd=40
    ecutwfc=70
    occupations = 'fixed'
    degauss=0.00

/

&ELECTRONS
    conv_thr    = 1.D-6
    mixing_beta = 0.07
/

ATOMIC_SPECIES
    K 39.09830 K.pz-hgh.UPF
    Nb 92.90638 Nb.pz-hgh.UPF
    O 15.99940 O.pz-hgh.UPF

ATOMIC_POSITIONS {crystal}
K        0.000000000   0.000000000   0.000000000
Nb      0.500000000   0.500000000   0.500000000
O        0.500000000   0.500000000   0.000000000
O        0.500000000   0.000000000   0.500000000
O        0.000000000   0.500000000   0.500000000

K_POINTS {automatic}
4 4 7  0 0 0







nscf.out


                             VALUES OF POLARIZATION
                             ~~~~~~~~~~~~~~~~~~~~~~

        The calculation of phases done along the direction of vector 3
        of the reciprocal lattice gives the following contribution to
        the polarization vector (in different units, and being Omega
        the volume of the unit cell):

           P =   3.8195000  (mod   7.6390000)  (e/Omega).bohr

           P =   0.0085683  (mod   0.0171367)  e/bohr^2

           P =   0.4898671  (mod   0.9797342)  C/m^2

        The polarization direction is:  ( 0.00000 , 0.00000 , 1.00000 )


               ==================================================
-- 
Regards,
*Rajender Tiwari*

*Ph.D. Scholar*
*Special Center for Nano Science*

*Jawaharlal Nehru University, New Delhi*
*#+91-9868970410/ 9873513329*
[image: Image result for jnu]
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