Polarisation of graphene with berry phase

[spaz]

hi all,

i am trying to calculate the polarisation change of 2D graphene sheet,which is coaxed with triangular hole to break the centrosymmetry of the structure, under applied stress. i relaxed the structure then applied the Berry's Phase procedure

LBERRY = .TRUE.
IGPAR = 3
NPPSTR = 6
DIPOL = 0.0 0.5 0.5

for applied stress in "z" direction (i squezzed and enlarged the z direction cell parameter in %0.01 and %0.02 ratios). by following the procedure in VASP manuscript , i got the polarisation values as follows;

strain Polarisation(eA)
0,02 4.2629
0,01 2.03086
0.00 0.00000
-0,01 2.08021
-0,02 4.16051


after then, i prepared 1x1x2 supercell of the system(which contains two triangular holes positioned in same direction) then applied the same procedure again, calculated polarisation values are as follows;


strain Polarisation(eA)
0,02 2.81835
0,01 1.39994
0.00 0.00000
-0,01 1.38627
-0,02 2.77240

i expected that the polarisations will be the same for the systems because of the symmetry,but there is 1.5 times difference.

what is the reason of the differences in the polarisation values.

structures.png

thanks for your replays.....

[Infant]

Does the 'z' direction means the direction where the strain is applied and parallel to that of plane (in-plane) as your arrow pointing in your illustration?