Light-absorption and luminescence processes in nano-sized materials can be modelled either by using computational approaches developed for quantum chemical calculations or by applying computational methods in the effective mass approximation (EMA) originally intended for solid-state theory studies. An overview of the theory and implementation of an ab initio correlation EMA method for studies of luminescence properties of embedded semiconductor quantum dots is presented. The applicability of the method and the importance of correlation effects are demonstrated by calculations on InGaAs/GaAs quantum-dot and quantum-ring samples. Ab initio and density functional theory (DFT) quantum chemical studies of optical transitions in freestanding silicon nanoclusters are also discussed. The accuracy of the optical gaps and oscillator strengths for silicon nanoclusters obtained using different computational methods is addressed. Changes in the cluster structures, excitation energies and band strengths upon excitation are reported. The role of the surface termination and functional groups on the silicon nanocluster surfaces is discussed.