There are three possible nearest neighbor N H = 2 clusters in bulk δ-Pu: neighboring occupied O sites (O2), neighboring occupied T sites (T2), and an occupied O site next to an occupied T site (T1O1). The O2 absorption energy ∆E bulk O2 is 0.25 eV (10 k B T ) greater than the energy of two independent O1 absorbates, 2∆E bulk O1 , so the equilibrium population of O2 configurations will be O(10 -5 ) of the absorbed hydrogen. The T1O1 and T2 absorption energies are even higher, 21 k B T and 27 k B T above the O1 absorption energy, respectively, indicating that the equilibrium population of either is negligible.