Fig. shows surface coverage θ as a function of ∆E. For ∆E = 0, surface sites will be populated in the same proportion as the bulk, θ = N bulk H /N bulk . As the surface adsorption energy increases (∆E becomes more positive), surface coverage drops exponentially. As the adsorption energy decreases (∆E becomes more negative), surface coverage increases but begins to plateau as maximum capacity is reached. For the smallest system size, N bulk = 10,000, there are only N H = 10 atoms so the maximum surface coverage is 10%. For systems with N bulk > 1 million sites, N H > N surf and we see that the two-state model is not artificially constrained to low θ. For ∆E = -10 k B T , the converged surface coverage is θ = 96%. This simplified two-state model of adsorption illustrates convenient guidelines to evaluate surface coverage at equilibrium. Sites with adsorption energies within ±2 k B T of the bulk absorption energy will be populated at approximately the bulk ratio, N H /N P u . Surface sites that are 10 k B T higher in energy than bulk adsorption will have negligible population. Sites that are 10 k B T lower in energy will be entirely filled. We can thus use ±10 k B T as an approximate bound for gauging how absorbed hydrogen atoms partition between surface and bulk sites. The first step in applying this framework to H surface coverage on δ-Pu is to compute bulk absorption energies, which is where we now turn.