Thus far, the GC-DFT has been mostly used for studying metallic electrodes while studies on SCEs remain limited. Yet, it has been shown that at least the CIP-DFT can model e.g. polaron formation, Fermi-pinning, and reaction thermodynamics and kinetics as a function of potential. However, all GC-DFT studies have used standard DFT slab models of the SCE and the more advanced atomistic models in such quantum continuum approximations have not been used but it is already known that SCEs require significantly thicker slab models than metallic systems. Initial tests have shown that an 8-layer SCE model does not provide a converged inner potential with respect to slab thickness. However, absolute convergence might not be required for GC-DFT to be useful and we have shown that a good agreement with experiments on the electrode potential at which surface polarons appear on TiO 2 can be reached by using a 4-layer slab model with CIP-DFT. Yet, the slow convergence with respect to slab thickness is to not only a numerical but also a theoretical issue as discussed in Ref. . In particular, the grand canonical and canonical ensembles may yield different expectation values for SCEs -this would mark a rare occurrence of ensemble non-equivalence, i.e.