[QE-users] QE7.0 Berry phase polarization calculation inconsistency and strange numeric results

[Aykut Turfanda]

I need to post it again because I forgot to add input files and the version
number.
Dear QE Users,

I am a PhD student working with Berry phase polarization calculations of
single layer 2D material hBN in quantum espresso (applying vacuum in the
z-direction).

I found -0.0000002 C/m^2 polarization in the z-direction in Quantum
ESPRESSO 7.0 without electric field; however, total phases belonging to
electronic and ionic contribution found as equal to each other and given as
0.40381 and -0.40381 with 5 significant digits.
Is it OK to take -0.0000002 C/m^2 as the polarization of the system? or Is
this a numerical error?

The material in the z-direction is not polar, is it possible that the
polarization operator for a non-polar material is ill defined for this
calculation? Should I use berry phase polarization for the polar materials
only?

Also, when I change the dimension of the unit cell in the z-direction and
position of the material inside the cell, this polarization value changes
even though I use the same material.

Most importantly, this polarization value changes with the number of
processors used with the mpirun -np. This is very strange, should I stick
to -np 1. I should not make it parallel?

I also saw that this polarization is given with microC/cm^2 in the papers.
Does this mean the last two digits from  " P =  -0.0000002  (mod
2.9386420)  C/m^2"  are not considered from the results obtained in the
Quantum ESPRESSO run?

Are all those numerical errors or physical phenomena because I know that
there are vacuum fluctuations and Lamb modes, and so on? Can we probe those
with QE?

I look forward to hearing from you soon.

Best regards,
Aykut Turfanda
PhD student at Istanbul Technical University


###############################
 SUMMARY OF PHASES
                               ~~~~~~~~~~~~~~~~~

                          Ionic Phase: -0.40381
                     Electronic Phase:  0.40381
                          TOTAL PHASE: -0.00000 MOD_TOT: 1


                             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 =  -0.0000030  (mod  40.7880378)  (e/Omega).bohr

           P =  -0.0000000  (mod   0.0514003)  e/bohr^2

           P =  -0.0000002  (mod   2.9386420)  C/m^2

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

################################
&control
calculation = 'nscf',
restart_mode='from_scratch'
prefix = 'bn'
outdir = './tempdir'
pseudo_dir = '~/pseudo_my/'
etot_conv_thr = 1d-05
forc_conv_thr =   1d-04
lberry=.true.
gdir          = 3
nppstr        = 10
/
&system
ibrav =  0,
nat =  2,
ntyp = 2,
ecutwfc = 80
occupations='fixed'
smearing='gaussian'
degauss=0.02
/
&electrons
 conv_thr =   1d-08
/
&ions
/
&cell
/
ATOMIC_SPECIES
B    10.811  B.pbe.nc.sr.upf
N    14.0067  N.pbe.nc.sr.upf
ATOMIC_POSITIONS angstrom
B            -0.0000000000        1.4480800000        1.6085249900
N             1.2540740000        0.7240400000        1.6085249900
K_POINTS automatic
    10 10 10 0 0 0
CELL_PARAMETERS angstrom
2.5081477165 0.0000000000 0.0000000000
-1.2540738583 2.1721196390 0.0000000000
0.0000000000 0.0000000000 21.5841000986
##############################
&control
calculation = 'scf',
restart_mode='from_scratch'
prefix = 'bn'
outdir = './tempdir'
pseudo_dir = '~/pseudo_my/'
etot_conv_thr = 1d-05
forc_conv_thr =   1d-04
/
&system
ibrav =  0,
nat =  2,
ntyp = 2,
ecutwfc = 80
occupations='fixed'
smearing='gaussian'
degauss=0.02
/
&electrons
 conv_thr =   1d-08
/
&ions
/
&cell
/
ATOMIC_SPECIES
B    10.811  B.pbe.nc.sr.upf
N    14.0067  N.pbe.nc.sr.upf
ATOMIC_POSITIONS angstrom
B            -0.0000000000        1.4480800000        1.6085249900
N             1.2540740000        0.7240400000        1.6085249900
K_POINTS automatic
    10 10 1 0 0 0
CELL_PARAMETERS angstrom
2.5081477165 0.0000000000 0.0000000000
-1.2540738583 2.1721196390 0.0000000000
0.0000000000 0.0000000000 21.5841000986
#################################
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