We have investigated several ways of introducing a solvated proton into a DFT calculation in order to mimic an electrochemical interface: an extra hydrogen introduced into a metal bilayer, a Zundel and an Eigen ion. In all these cases the charge on the supposed proton is substantially less than a unit positive charge. In contrast, when the electrode is represented as a cluster, the charge on Zundel ion is indeed plus one. However, the distribution of the compensating charge on the cluster is quite different from that on a plane metal surface. (c) 2013 Elsevier Ltd. All rights reserved.