Monte Carlo simulations and cluster-exact approximation applied to H/Cu(100), H/Ag (100) and O/Cu(100) systems

The systems H/Cu(100), H/Ag (100) and O/Cu(100) were analyzed theoretically by means of DFT calculations, Monte Carlo (MC) simulations and a Cluster-exact (CA) Approximation. To model these real systems, a two stages procedure was used. In a first stage, DFT calculations were performed in order to determine the adsorption energies corresponding to a hydrogen or oxygen atom in different environments. The obtained values depend on the number of first neighbors present in each adsorption site. For the systems H/Cu(100) and O/Cu(100), lateral interactions between adatoms were found to be repulsive, while for the H/Ag (100) system, the interactions are attractive, except for the case of four nearest neighbors. Then, with this information, MC simulations and CA approximation were applied for different temperatures. The process was monitored by following the surface coverage as a function of the chemical potential (adsorption isotherm). Interesting behaviors were observed and discussed in terms of the low-temperature phases formed in the adsorbed layer. In addition, a good agreement was found between MC and CA results, especially at high temperatures. At low temperatures, a chessboard-like phase was observed for the systems H/Cu(100) and O/Cu(100).