  Periodic supercell models of electric double layers formed at the interface
between a charged surface and an electrolyte are subject to serious finite size
errors and require certain adjustments in the treatment of the long-range
electrostatic interactions. In a previous publication (C. Zhang, M. Sprik,
Phys. Rev. B 94, 245309 (2016)) we have shown how this can be achieved using
finite field methods. The test system was the familiar simple point charge
model of a NaCl aqueous solution confined between two oppositely charged walls.
Here this method is extended to the interface between the (111) polar surface
of a NaCl crystal and a high concentration NaCl aqueous solution. The crystal
is kept completely rigid and the compensating charge screening the polarization
can only be provided by the electrolyte. We verify that the excess electrolyte
ionic charge at the interface conforms to the Tasker 1/2 rule for compensating
charge in the theory of polar rocksalt (111) surfaces. The interface can be
viewed as an electric double layer with a net charge. We define a generalized
Helmholtz capacitance $C_\text{H}$ which can be computed by varying the applied
electric field. We find $C_\text{H} = 8.23 \, \mu \mathrm{Fcm}^{-2}$, which
should be compared to the $4.23 \, \mu \mathrm{Fcm}^{-2}$ for the (100)
non-polar surface of the same NaCl crystal. This is rationalized by the
observation that compensating ions shed their first solvation shell adsorbing
as contact ions pairs on the polar surface.
