The aim of this work is to investigate the structural, electronic and optical properties of hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations. Recent results about ab-initio calculations of Stokes shift as a function of the cluster dimension and of optical gain will be presented here. A structural model that can be linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be suggested too.