  The Ewald3D sum with the tinfoil boundary condition (e3dtf) evaluates the
electrostatic energy of a finite unit cell inside an infinitely periodic
supercell. Although it has been used as a {\it de facto} standard treatment of
electrostatics for simulations of extended polar or charged interfaces, the
finite-size effect on simulated properties has yet to be fully understood.
There is, however, an intuitive way to quantify the average effect arising from
the difference between the e3dtf and Coulomb potentials on the response of
mobile charges to contact surfaces with fixed charges and/or to an applied
external electric field. While any charged interface formed by mobile
countercharges that compensate the fixed charges slips upon the change of the
acting electric field, the distance between a pair of oppositely charged
interfaces is found to be nearly stationary, which allows an analytic
finite-size correction to the amount of countercharges. Application of the
theory to solvated electric double layers (insulator/electrolyte interfaces)
predicts that the state of complete charge compensation is invariant with
respect to solvent permittivities, which is confirmed by a proper analysis of
simulation data in the literature.
