Top panel: contour map of the potential energy surface as a function of the H position on the graphite. The energy difference between two consecutive contour lines is 0.3 eV. A, B, C and D are the positions of a barrier between two chemisorption sites. O is the center of the hexagon. H atoms can chemisorb and physisorb on top of carbon atoms (A, B, C and D), while they can physisorb also in other locations such as a bridge (E). Bottom panel: stable configuration for the second H atom: ortho (closer H atom on the hexagon), meta (M) and para (opposite H atom on the hexagon) configurations and 5 and S (second cycle, Dumont et al. 2008)