A theoretical model for the dielectric permittivity of nematics has been recently proposed [1], based on the atomistic representation of a probe molecule interacting with a medium which is characterised by its macroscopic properties. Electrostatic interactions are described through the classical model of a charge distribution contained in a molecular- shaped cavity embedded in an anisotropic dielectric continuum. Short-range intermolecular interactions are parameterized in terms of the anisometry of the molecular surface, which is defined according to the "rolling sphere" representation. The results obtained for the isotropic and nematic phases of 4,4'-pentyl-cyanobiphenyl and 4,4'-pentylcyanobicyclohexyl are reported; a good agreement with experiment appears, with a significant improvement with respect to the Maier-Meier theory.