  The absorption energy of atomic hydrogen at rotated graphene bilayers is
studied using ab initio methods based on the density functional theory
including van der Waals interactions. We find that, due to the surface
corrugation induced by the underneath rotated layer and the perturbation of the
electronic density of states near the Fermi energy, the atoms with an almost AA
stacking are the preferential ones for hydrogen chemisorption. The adsorption
energy difference between different atoms can be as large as 80 meV. In
addition, we find that, due to the logarithmic van Hove singularities in the
electronic density of states at energies close to the Dirac point, the
adsorption energy of either electron or hole doped samples is substantially
increased. We also find that the adsorption energy increases with the decrease
of the rotated angle between the layers. Finally, the large zero point energy
of the C-H bond (${\sim 0.3 eV}$) suggests adsorption and desorption of atomic
hydrogen and deuterium should behave differently.
