  Tasker type III polar terminations of ionic crystals carry a net surface
charge as well as a dipole moment and are fundamentally unstable. In contact
with electrolytes, such polar surfaces can be stabilized by adsorption of
counter ions from solution to form electric double layers (EDLs). In a previous
work (J. Chem. Phys 147, 104702 (2017)) we reported on a classical force field
based molecular dynamics study of a prototype model system namely a NaCl(111)
slab interfaced with an aqueous NaCl solution on both sides. A serious hurdle
in the simulation is that the finite width of the slab admits an electric field
in the solid perturbing the theoretical charge balance at the interface of
semi-infinite systems (half the surface charge density for NaCl(111)). It was
demonstrated that the application of a finite macroscopic field $E$ cancelling
the internal electric field can recover the correct charge compensation at the
interface. In the present work, we expand this method by applying a conjugate
electric displacement field $D$. The benefits of using $D$ instead of $E$ as
the control variable are two fold: it does not only speed up the convergence of
the polarization in the simulation but also leads to a succinct expression for
the biasing displacement field involving only structural parameters which are
known in advance. This makes it feasible to study the charge compensating
phenomenon of this prototype system with density functional theory based
molecular dynamics (DFTMD), as shown in this work.
