Silicon nanoclusters have been attracted in recent decades because of their applications in different industries such as optoelectronic, microelectronic, solarcells and medical industry. Because optical gap of silicon nanoclusters (quantum dots) are tunable by their sizes, they are interesting. We can mention photoluminescens in room temperature as another optical properties of silicon nanoclusters. As we know optical propertie including absorption energy(optical gap) and emission energy, and because in this project we consider application of these type of clusters in solar cells, hence we will investigate their absorbtion spectrum and calculate their optical gap.Absorption spectrum of Silicon nanoclusters are depend on their geometry, size of clusters and passivation of dangling bonds. In most works bulk-like structures have been considered dangling were passivated bonds by different elements like: hydrogen,Oxygen,Fluorin and etc. Confinement effect has been investigated for bulk-like structures and according to it, optical gap of bulk-like structures de-creases by increasing the size of clusters.In this project we want to consider structures which obtained by a sophisticated method,evolutionary algorithm by USPEX package. For these structures we in-vestigate absorption spectrum and optical gap. As we know density functional theory is a ground state theory and for calculation of absorption spectrum , system should be considered under external perturbation(like electromagnetism field) hence we have to use time-dependent theory. Hence we use different methods for calculation of optical gap. At first step, optical gap of structures are calculated by TD DFTB method and then optical gap are calculated by turbo TDDFT and TD DFT to increase the accuracy of computations.