[Pw_forum] Negative value of polarization for doped BaTiO3

[Deepashri Saraf]

Dear Users,

I am trying to perform berry phase calculations on pristine as well as
doped BaTiO3 (tetragonal). I get reasonable results for the value of
polarization for pristine BaTiO3.

------------------------------------------------------------------------------------------------------------------------
                              SUMMARY OF PHASES
                               ~~~~~~~~~~~~~~~~~

                          Ionic Phase: -0.12600 (mod 2)
                     Electronic Phase:  0.40671 (mod 2)
                          TOTAL PHASE:  0.28071 (mod 2)


                             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 =   2.1352035  (mod  15.2130988)  (e/Omega).bohr

           P =   0.0049373  (mod   0.0351775)  e/bohr^2

           P =   0.2822719  (mod   2.0111573)  C/m^2

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

------------------------------------------------------------------------------------------------------------------------

But in case of doped BaTiO3, I get negative value of polarization. I used a
2x2x3 supercell for these calculations.

-------------------------------------------------------------------------------------------------------------------------

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

                          Ionic Phase: -0.50398 (mod 2)
                      Electronic Phase: -0.37438 (mod 2)
                          TOTAL PHASE: -0.87836 (mod 2)


                             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 = -19.9950561  (mod  45.5280337)  (e/Omega).bohr

           P =  -0.0038804  (mod   0.0088355)  e/bohr^2

           P =  -0.2218486  (mod   0.5051413)  C/m^2

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

I also tried using a finer k-mesh, but landed up with the same value of
polarization. Does the negative polarization have any physical meaning or
is it a numerical error?

Any help appreciated. Thanks in advance.

Best,
DS.

-- 
Deepashri Saraf
PhD Student
Department of Physics, SPPU
Pune (India)
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