We discuss how an approach that combines global optimisation and time-dependent density functional theory (TD-DFT) calculations allows one to predict the photoluminescence (PL) signature of true nanosized semiconductor nanoparticles. As a demonstration we calculate the PL signature of both bare and water covered zinc sulfide nanoparticles and demonstrate that we can successfully reproduce their experimentally measured PL signatures. Our TD-DFT calculations suggest that the excited state, after relaxation, becomes highly localised and that the degree of localisation changes with nanoparticle size. We also show that adsorbed water molecules can strongly influence the nanoparticle's final excited state minimum and PL signature. Finally, we discuss how this approach can be used to predict the effect of doping and what the next important methodological step will be on the route to theoretically understanding processes such as photocatalysis.