To assess the intrinsic ability of a rutile (110) surface to evolve lattice oxygen, it is useful to calculate oxygen binding energies at the surface. The system is modelled by a computational slab such as the one shown in fig. . Employing DFT one can determine the binding energy of oxygen bound in positions 1 through 9 while varying the local structure by substituting metal cations in positions A through L. Positions 1, 3 and 5 are commonly referred to as the bridge sites in the rutile structure. Oxygen bound here is singly undercoordinated compared to that of the lattice, but is usually considered inactive. Positions 2, 4, 6 are the coordinatively unsaturated sites (CUS). Oxygen bound here are doubly undercoordinated compared to the lattice and considered active. This surface site is generally considered to be an active site for Volmer-Heyrovsky type single site oxygen evolution on rutile (110) surfaces . The oxygen binding energy (∆E O ) is calculated as the energy change associated with going from a water molecule and a surface to a hydrogen molecule and a surface with the adsorbed oxygen: