  A thorough understanding of the electronic structure is a necessary first
step for the design of nanoelectronics, chemical/bio-sensors, electrocatalysts,
and nanoplasmonics using graphene. As such, theoretical spectroscopic
techniques to describe both direct optical excitations and collective
excitations of graphene are of fundamental importance. Starting from density
functional theory (DFT) we use the time dependent linear response within the
random phase approximation (TDDFT-RPA) to describe the loss function
-Im{{\epsilon}\overline{ }^{1}(q,{\omega})} for graphene. To ensure any
spurious interactions between layers are neglected, we employ both a radial
cutoff of the Coulomb kernel, and extra vacuum directly at the TDDFT-RPA level.
