The iodine vacancy was investigated in the charge states 1-, 0 and 1+, following reports from previous computational studies of a 1 + ∕1-transition level. The ground state of the neutral vacancy is found by a 20% bond distortion, shown in Figure , and sees an opposite pair of Cu atoms relax into the vacancy, while another pair relax away from it. The structure obtained from the "unperturbed" relaxation is significantly different, shown in Figure , consisting of an open cage around the vacancy, and then migration of the remaining electron to an adjacent I atom, subsequently causing an inward relaxation of four Cu atoms, and is 100 meV higher in energy. It is possible that this higher energy structure is the one identified by previous studies, pushing the neutral defect higher in energy on the transition level diagram and facilitating the negative-U behaviour. The fully ionised ground state iodine vacancy is shown in Figure , and is a simple outward relaxation of the Cu atoms around the vacancy, caused by the removal of a centre of negative charge. This example demonstrates the importance of sampling the potential energy surface thoroughly, although admittedly for the iodine vacancy the formation energies are so high that these differences in defect geometry play little to no rôle in the overall defect chemistry of CuI.