  The adsorption of an adenine molecule on graphene is studied using a
first-principles van der Waals functional (vdW-DF) [Dion et al., Phys. Rev.
Lett. 92, 246401 (2004)]. The cohesive energy of an ordered adenine overlayer
is also estimated. For the adsorption of a single molecule, we determine the
optimal binding configuration and adsorption energy by translating and rotating
the molecule. The adsorption energy for a single molecule of adenine is found
to be 711 meV, which is close to the calculated adsorption energy of the
similar-sized naphthalene. Based on the single molecular binding configuration,
we estimate the cohesive energy of a two-dimensional ordered overlayer. We find
a significantly stronger binding energy for the ordered overlayer than for
single-molecule adsorption.
