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# LCALCPOL

LCALCPOL = .TRUE. | .FALSE.
Default: LCALCPOL = .FALSE.

Description: LCALCPOL=.TRUE. switches on the evaluation of the Berry phase expressions for the macroscopic electronic polarization in accordance with the so-called Modern Theory of Polarization.

For LCALCPOL=.TRUE., VASP calculates the electronic contribution to the polarization, along the three reciprocal lattice vectors Gi, i=1,2,3, (i.e. Σi P·Gi) in a single run (unlike LBERRY=.TRUE.).

### An example: The fluorine displacement dipole (Born effective charge) in NaF

PREC = Med
EDIFF= 1E-6

ISMEAR = 0
DIPOL  = 0.25 0.25 0.25

LCALCPOL = .TRUE.

6x6x6
0
Gamma
6 6 6
0 0 0

NaF
4.5102
0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
1 1
Direct
0.0000000000000000  0.0000000000000000  0.0000000000000000
0.5000000000000000  0.5000000000000000  0.5000000000000000

• and LDA Na_sv and F PAW datasets.

The OUTCAR file should now contain the following lines:

            Ionic dipole moment: p[ion]=(     2.25510     2.25510     2.25510 ) electrons Angst

Total electronic dipole moment: p[elc]=(     0.00000     0.00000     0.00000 ) electrons Angst


Here the units "electrons Angst" denote ${\displaystyle e\mathrm {\AA} =-1.60210^{-19}C\mathrm {\AA} }$.

To calculate the change in the electronic polarization of NaF due to the displacement of the fluorine sublattice we repeat the previous calculation with the following POSCAR file:

NaF
4.5102
0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
1 1
Direct
0.0000000000000000  0.0000000000000000  0.0000000000000000
0.5100000000000000  0.5100000000000000  0.4900000000000000


The OUTCAR should now contain something very similar to the following lines:

            Ionic dipole moment: p[ion]=(     2.25510     2.25510     1.93939 ) electrons Angst

Total electronic dipole moment: p[elc]=(     0.00000     0.00000     0.36061 ) electrons Angst


From the above one easily recognizes that the change in the electronic dipole moment due to the F-sublattice displacement is:

${\displaystyle \Delta \mathrm {p[elc]} =0.3606{\hat {z}}\;e\mathrm {\AA} }$

and the corresponding change in the ionic dipole moment:

${\displaystyle \Delta \mathrm {p[ion]} =1.93939-2.25510=-0.31571{\hat {z}}\;e\mathrm {\AA} }$

Thus the total change is found to be:

${\displaystyle \Delta \mathrm {p[tot]} =0.36061-0.31571=0.0449{\hat {z}}\;e\mathrm {\AA} }$

and considering that the F-sublattice was displaced by 0.045102 Å these calculations yield a Born effective charge for fluorine of

${\displaystyle Z^{*}=0.0449/0.045102=-0.995|e|\;}$.

The socalled parallel or ${\displaystyle {\mathbf {G}}_{\parallel }}$ direction in the integration over the reciprocal space unit cell is set in IGPAR.