Theoretical gas-surface models that describe adsorption over a wide range of coverages can provide qualitative insight into chemical phenomena that occur at intermediate to high coverages, such as subsurface adsorption, surface reconstruction, and industrial heterogeneous catalysis. However, most atomistic, quantum-mechanical models of gas-surface adsorption are limited to low adsorbate coverage due to the large computational cost of models built using many surface atoms and adsorbates. To investigate adsorption in the subsurface of a crystalline solid with increasing coverage, we present a lattice-gas adsorption model that includes surface and subsurface sites of the solid, and is fully parametrized using density functional theory. We apply the model to study the competition between surface and subsurface adsorption of atomic oxygen on the Ag(111) surface. Oxygen population distributions calculated using the model show the onset of subsurface adsorption at a total coverage of approximately 1/4 monolayer and a greater accumulation of oxygen in the second rather than the first subsurface at total coverages greater than 1/2 monolayer. Computation of core-electron binding energies and projected density of states of an oxygen distribution predicted by the model reveal qualitative differences in oxygen-silver bonding at the surface and subsurface, suggesting that oxygen adsorbed in the two regions could play distinct roles in surface chemistry.
The supplementary information contains images of surface sites, parameters of the adsorption model, and a parity plot comparing the model to DFT.