[QE-users] Berry phase polarization results

[Aykut Turfanda]

Dear QE users,

I am a PhD student working with Berry phase theory of 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 from lberry=.true. in QE; however,
total phases belonging to electronic and ionic contributions are 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?

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