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Formation enthalpies and migration barriers of copper interstitials and Frenkel pairs in CuInSe₂ (CIS) are determined by first-principles calculations within density functional theory using the nonlocal screened exchange Heyd-Scuseria-Ernzerhof (HSE06) functional. Interstitials occur on four symmetrically inequivalent sites with formation enthalpies of 0.17–0.38 eV, which are much lower than previously reported values based on local approximations. A direct interstitial and indirect interstitialcy diffusion mechanism with migration barriers as low as 0.22 and 0.34 eV are identified. The results provide evidence that the fast interstitial diffusion of copper is important for understanding metastabilities, Fermi-level pinning at interfaces, electric-field-induced creation of p-n junctions, and widely varying experimentally measured diffusion coefficients in CIS devices.