where e bulk P u is the energy of a plutonium atom in a defect-free fcc lattice, e H is the energy of a hydrogen atom in a gas-phase hydrogen molecule, and N P u and N H are the number of plutonium and hydrogen atoms in configuration i, respectively. The absorption energy reveals that it is energetically favorable for an isolated hydrogen atom in bulk δ-Pu to absorb at an O site (∆E bulk O1 = -2.92 eV) rather than at a T site (∆E bulk T 1 = -2.71 eV). The difference ∆E bulk T 1 -∆E bulk O1 = 0.21 eV (8 k B T at room temperature) and the corresponding Boltzmann factor, exp(-8), shows that the fraction of absorbed hydrogen occupying T sites is O(10 -4 ).