Density functional theory study of the electronic structure of fluorite Cu2Se

We have investigated the electronic structure of fluorite Cu2Se using density functional theory calculations within the LDA, PBE and AM05 approximations as well as the non-local hybrid PBE0 and HSE approximations. We find that Cu2Se is a zero gap semiconductor when using either a local or semi-local density functional approximation while the PBE0 functional opens up a gap. For the HSE approximation, we find that the presence of a gap depends on the range separation for the non-local exchange. For the occupied part in the density of states we find that LDA, PBE, AM05, PBE0 and HSE agree with regard to the overall electronic structure. However, the hybrid functionals result in peaks shifted towards lower energy compared to LDA, PBE and AM05. The valence bands obtained using the hybrid functionals are in good agreement with experimental valence band spectra. We also find that the PBE, PBE0 and HSE approximations give similar results regarding bulk properties, such as lattice constants and bulk modulus. In addition, we have investigated the localization of the Cu d-states and its effect on the band gap in the material using the LDA + U approach. We find that a sufficiently high U indeed opens up a gap; however, this U leads to valence bands that disagree with experimental observations.