In this paper we study the relations between the structure of approximately spherical carbonaceous nanoparticles and their frequency dependent dielectric susceptibility in the optical and near-UV domain. Because the nanoparticles are defined atomistically, the set of atomic point dipoles is distributed on an irregular grid and we cannot use traditional discrete-dipole approximation (DDA) codes. We thus developed an approximate multi-step methods that allowed us to discuss scattering of light by both carbonaceous nanoparticles and aggregates of these nanoparticles. We find that the dielectric susceptibility of these nanoparticles is highly dependent on the type of chemical units used to construct the particles, but almost independent of their distribution in the various shells of the nanoparticle.