Clearly the V Cu species is the principal defect in CaCuP, and we can now look at it in greater detail. Figure shows the partial hole density of V Cu . The charge density is delocalised through- out the Cu-P layers in the crystal structure, across the same orbitals that comprise the valence band maximum. Therefore, we expect high hole mobility in two directions, as predicted by the charge transport calculations, and as indicated by the highly disperse bands in the Γ → M and Γ → K directions in Figure . † The delocalised nature of this hole is in stark contrast with copper vacancies in p-type transparent conducting oxides. For example, in CuAlO 2 the hole generated by V Cu is partially localised on the six surrounding Cu atoms, indicative of polaronic behaviour, rather than throughout the whole material. Similarly, in Cu 2 O, the hole density is localised on the four surrounding O atoms.