Quasiclassical trajectory studies have been performed for the reaction between an H (or D) atom incident from the gas phase and a H (or D) atom adsorbed onto a Cu(111) surface. Results from a density functional calculation of the interaction between H and a Cu(111) surface are used to construct a detailed potential energy surface which contains all six nuclear degrees of freedom. Impacts of the incident atom close to the adsorbate can lead to direct Eley-Rideal reactions and the dynamics of these reactions are explored. Interaction of the incident atom with the adsorbate also results in trapping, with a high probability. This adsorbate-mediated trapping mechanism is important for impacts within 2 Angstrom of the adsorbate. At larger impact parameters scattering from the corrugation also leads to trapping. These trapped ''hot'' atoms can go on to react with an adsorbed species, and the dynamics of such hot-atom reactions are explored. The final-state distributions of the products are examined with regard to isotope effects for the direct and hot-atom pathways, and compared with experiment. (C) 1997 American Institute of Physics.