  We investigated binding of hydrogen atoms to small Polycyclic Aromatic
Hydrocarbons (PAHs) - i.e. graphene dots with hydrogen-terminated edges - using
density functional theory and correlated wavefunction techniques. We considered
a number of PAHs with 3 to 7 hexagonal rings and computed binding energies for
most of the symmetry unique sites, along with the minimum energy paths for
significant cases. The chosen PAHs are small enough to not present radical
character at their edges, yet show a clear preference for adsorption at the
edge sites which can be attributed to electronic effects. We show how the
results, as obtained at different level of theory, can be rationalized in
detail with the help of few simple concepts derivable from a tight-binding
model of the $\pi$ electrons.

Mesoscale and Nanoscale Physics