The electronic structure of spherical Cd(m),S(n) nanoparticles having zinc-blende symmetry and the diameters of up to around 3 nm has been studied by Hartree-Fock theory to find out the effect of the cluster size on the optical energy gap between HOMO and LUMO. The effect of encapsulation on the electronic structure has been also investigated for CdS4 and Cd13S4 clusters embedded in SiO2 matrix and sodalite cage by Hartree-Fock theory. It was found that the energy gap of CdS nanoparticles can be regulated by both the cluster size and the interface provided by the SiO2 matrix and sodalite cage. The energy gap between HOMO and LUMO has been found to be increased to upper boundary of the visible spectra when CdS4 and Cd13S4 clusters have been embedded in either SiO2 matrix or sodalite cage.