Monkhorst-Pack mesh was used for all bulk calculations, whereas we used a mesh of 5×5×1 for our surface calculations, with the value of one aligned with the surface normal. We have used the GGA+U approach of Dudarev et al., as implemented in VASP. The value of the effective on-site Hubbard correction was U -J = 1.3 eV (U = 2.05 eV and J = 0.75 eV) for δ-Pu 6 and U -J = 2.1 eV (U = 2.85 eV and J = 0.75 eV) for PuH 2 . Hydrogen atoms were placed in octahedral or tetrahedral sites, after which ionic positions were relaxed using the conjugate gradient algorithm until forces were less than 0.05 eV/Å. Cell size and shape were optimized under zero pressure.