Energetics and structures of neutral and charged Si n ͑nр10͒ and sodium-doped Si n Na clusters have been investigated using local spin density functional electronic structure calculations and structural optimizations, with and without exchange-correlation gradient corrections. For the Si n clusters, the monomer separation energies show local maxima for nϭ4, 7, and 10. The vertical and adiabatic ionization potentials are smaller than the values for the Si atom and exhibit odd-even oscillations with values in agreement with experiments, and the adiabatic electron affinities show local minima for nϭ4, 7, and 10, with the value for the heptamer being the smallest, in agreement with the experimentally measured pattern. Binding of Na to Si n is characterized by charge transfer from the sodium resulting in the development of significant dipole moments for the Si n Na clusters. The binding energy of Na to Si n oscillates as a function of n, with local maxima for nϭ2, 5, and 9, and local minima for nϭ4, 7, and 10, with the value for nϭ7 being the smallest. A similar trend is found for the vertical and adiabatic ionization potentials of the doped clusters, correlating with the electron affinity trend exhibited by the Si n clusters, and in agreement with recent measurements. In the optimal adsorption geometry of H 2 O on the Si 7 Na cluster, the oxygen is bonded to the Na, with a hydration energy significantly higher than that of an isolated sodium atom. The vertical and adiabatic ionization potentials of NaH 2 O are lower than those of Si 7 NaH 2 O, and the values for the latter are lower, by Ϸ0.2 eV, than those of the unhydrated Si 7 Na cluster. ͓S0163-1829͑97͒05211-9͔