Figure 1: Schematic diagram of possible reconstruction processes. (a) Stacking of $-1$ and $+1$ layers, where $E$ is the electric field and $-V$ the electrostatic potential. In this case, the potential diverges since there is no reconstruction. (b) A structure in which the top layer of the V oxide portion of the system reconstructs to a charge $-1/2$. The electrostatic potential does not diverge in this case. (c) Another example where there is no divergence. In this case, the surface is reconstructed to a charge $-1/2$. The competition between (b) and (c) is the key observation reported by Higuchi et al. [2].