The substitutional defects themselves are higher in energy than the copper vacancy under ptype growth conditions (Figures and), at around 0.9 eV for S I and 1.0 eV for Se I , and have deeper transition levels. This suggests a greater degree of charge localisation than for the copper vacancy, which is shown in Figure . The hole generated by the formation of both defects is strongly bound to the site of the substitution, and causes a significant inward relaxation of the four surrounding copper atoms. This deep transition level is supported by the experimental work of Storm et al., who report a binding energy for the Se substitution of around 300 meV. Graužinyt ė et al. predict shallower transition levels for both S I and Se I , but these are from defect relaxations performed with a semi-local exchange-correlation functional without SOC, leading to an over-delocalisation of charge and an incorrect defect geometry. (a) S 0 I , 0.006 e Å Finally, the interstitial chalcogen species are much higher in formation energy, and are predicted to act as ultra-deep acceptors, with negative-U 0∕2-transitions occurring at Fermi levels close to 2.0 eV. These characteristics preclude them from having any significant effect on the electronic properties of CuI.