  We use a configuration interaction approach within the envelope function
approximation to study the nature of the excitonic resonance in nano-hybrids,
composite nanoparticles (NPs) combining a semiconducting and a metallic segment
in contact. With reference to recent experimental reports, we specifically
study CdS-based nanorods with metallic NPs deposited at the tips (matchstick)
or metallic coatings (core-shell). The excitonic states are computed taking
into account both the renormalization of the electron-hole interaction and
self-energy effects induced by the the metallic segment on the electron-hole
pair, as well as by the dielectric environment, through an induced charge
numerical approach. In neutral matchstick structures the metal NP has only a
minor influence (~1 meV) on the excitonic states. When the metallic NP is
charged the exciton becomes rapidly redshifted and spatially indirect. In
contrast, in neutral core-shell structures the exciton energy redshifts by tens
of meV
