Quantum simulation methods, such as density functional theory (DFT), are necessary to model the bond breaking inherent in hydrogen dissociation and adsorption. DFT remains one of the most popular modeling methods in condensed matter physics, computational chemistry, and materials science, and has been widely used in studies of the physical and structural properties of δ-Pu (e.g., Refs. 7,8). While a comprehensive explanation of plutonium's magnetism has yet to be offered, plutonium does not exhibit a bulk magnetic moment and anti-ferromagnetic (AFM) models yield structural properties comparable to experiments. Surface hydriding studies are generally framed in terms of low (single molecule per surface unit) vs. high (nearing full monolayer) coverage. Low coverage conditions will be expected at hydrogen partial pressures P H 2 well below the equilibrium pressure P eq H 2 , P H 2 P eq H 2 . Previous DFT studies of adsorption of a single H atom or H 2 molecule on clean (100) and (111) surfaces of δ-Pu have all been conducted at low coverage conditions. High surface coverage will be expected when P H 2 P eq H 2 . The equilibrium pressure P eq H 2 at room temperature, has not been experimentally verified but was recently predicted from simulations to be O ) bar. Even very small pressures (e.g. 1 mbar) will exceed P eq H 2 by many orders of magnitude resulting in rapid adsorption and, unless the surface has a protective layer of PuO 2 , barrierless hydriding. Immediate hydriding at millibar pressures has been observed by experiments. We should therefore anticipate that under real world conditions, adsorption onto the surface from the gas phase may occur more quickly than surface adatoms can burrow from the surface into the bulk. However, to date there have been few systematic studies of Pu surface hydriding including determination of initial mechanisms as well as studies of middle-ranged surface coverage and how hydrogen can accumulate on a given facet.