Copper selenide (Cu 2−x Se) is a promising material for plasmonic nanoparticle applications. Cu 2−x Se becomes plasmonically active in the oxidized state (x > 0), where free carrier density increases with increasing oxidation. It also has considerable cation (Cu) disorder. To date, there has not been a theoretical study of the impact of the degree of oxidation and cation disorder on the electronic and optical properties of Cu 2−x Se. We used density functional theory (DFT) to investigate the effects of the concentration of Cu vacancies and disorder on the properties of Cu 2−x Se. We used both generalized gradient approximation (GGA) and hybrid-GGA functionals to compute the structural, electronic, and optical properties of the cubic phase of Cu 2−x Se for x = 0, 0.25, 0.5, and 0.75. We performed ab initio molecular dynamics simulations at 300 K to simulate disorder, taking snapshots sampled from simulations of periodic supercells with different levels and arrangements of defects. The HSE06+U hybrid-GGA functional was used to calculate the electronic properties, including the optical band gaps, for a total of 400 different configurations. We found that the average optical band gap increases with increasing oxidation. We also found that only the stoichiometric, x = 0, materials are semiconductors, and the electronic band gap generally increases, increasing disorder.