Optimized geometries and electronic structures of hydrogenated silicon nanoclusters, which include the T-d and I-h symmetries, have been generated by using the semiempirical AM1 and PM3 methods, the density functional theory (DFT) B3LYP method with the 6-31 G(d) and LANL2DZ basis sets from the Gaussian 03 package, and the local density functional approximation (LDA), which is implemented in the SIESTA package. The calculated diameters for these Td symmetric hydrogenated silicon nanoclusters are in the range from 6.61 angstrom (Si5H12) to 23.24 angstrom (Si281H172). For the I-h symmetry, we calculated Si20H20 and Si100H60 nanoclusters only. Theoretically, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is size dependent. The calculated energy gap decreases (Si5H12: 7.65 eV to Si281H172: 3.06 eV) while the diameter of silicon nanocluster increases. By comparing different calculated results, we concluded that the calculated energy gap by B3LYP/6-31G(d)//LDA/SIESTA is close to that from experiment and that the LDA/SIESTA result underestimates the experimental value. On the contrary, the AM I and PM3 results overestimate the experimental results. For investigation of the optical properties of Si nanoclusters as a function of surface passivation, we carried out a B3LYP/6-31G(d)//LDA/SIESTA calculation of the Si-35 and Si-47 core clusters with full alkyl-, OH-, NH2-, CH2NH2-, OCH3-, SH-, C3H6SH-, and CN- passivations. The calculated optical properties of alkyl passivated Si-35 nanoclusters (Si-35(CH3)(36), Si-35(C2H5)(36), and Si-35(C3H7)(36)) are close to one another and are higher than those of oxide, nitride, and sulfide passivated Si-35 clusters. In conclusion, the alkyl passivant affects weakly the calculated optical gaps, and the electron-withdrawing passivants-generate a red-shift in the energy gap of silicon nanoclusters. A size-dependent effect is also observed for these passivated Si nanoclusters.