**4. Computational Detail**

#### *4.1. Interatomic Potentials Calculations*

We have used the semi-classical GULP code [27] to construct and optimise ZnO structures prior to refining them with DFT. We employed polarisable shell inter-atomic potentials parameterised for bulk ZnO [7,28] in the double bubble cluster and framework calculations. The resulting atomic structures were used not only for ZnO, but also GaN and mixed ZnO/GaN structures; note that the bond lengths in GaN are very similar those in ZnO (see Tables 1–3 in Section 3), and we only required approximate initial atomic coordinates for input into the DFT calculations, as outlined below.
