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. .