where i and j are different hydrogen configurations. Hydrogen atoms in an LB1H1 dimer are 1.8 Å apart with an interaction energy ∆∆E (110),int LB1H1 = 0.93 eV. In other words, hydrogen atoms adsorbed on nearest neighbor LB and H sites are nearly 1 eV higher in energy than hydrogen adsorbed on distant LB and H sites. In LB1fcc1 and LB1SB1, the hydrogen atoms are 2.1 and 3.4 Å apart with ∆∆E int of 0.33 and 0.10 eV, respectively, showing that the repulsive interaction decays with increasing distance.