The bulk region of a SCE is only reached after the surface charge has been completely screened by the SCR and this SCR significantly complicates the DFT modelling SCEs. In particular, because the space charge thickness for a typical doping concentration of 10 17 dopants/cm 3 is 100µm, extremely large slab models and simulation cells are needed to capture the SCR. This is particularly so for simulating the electrode potential of SCEs because the applied electrode potential is defined within the electrode bulk and measured from the backside of the electrode as was stressed in the previous section;