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Accurate ab initio molecular-orbital calculations are performed to obtain the ground-state geometries and electronic configurations of the silicon clusters ${\mathrm{Si}}_{\mathrm{n}}$ for n=2--7,10 and ${\mathrm{Si}}_{\mathrm{n}}^{+}$ for n=2--6. The effects of polarization functions and electron correlation are included in these calculations. All the optimized structures are considerably reconstructed from those derived from microcrystal geometries. Ionization potentials and binding energies are calculated and used to interpret the distribution and fragmentation of small silicon cluster ions observed recently.