An efficient method for the theoretical investigation of optical properties of semiconductor core shell quantum dots (CSQDs) is introduced within the multiband k.p approach, which takes the advantage of the symmetry of the system. The heteroepitaxial strain and excitonic effect are included in the calculation of energy levels, envelope wave functions, exciton binding energy, and linear absorption coefficient. The adoption of symmetrized orbitals allows improvement of the computation time significantly. To avoid appearance of spurious solutions caused by imbalance of basis functions adopted, we consider an 8-band k.p model which is block-diagonalized into two conduction bands and six valence bands, that we call the 2 + 6-band model. The band nonparabolicity effect is modeled by an energy-dependent k.p term, such that the density of states obtained can mimic the model. The simulated absorption spectra of ZnTe/ZnSe CSQD are in good agreement actual density of states of a full-band absorption at energies far above the absorption edge.