AbstractThe method of local pseudopotentials is applied to the calculation of several ground‐ and excited‐state properties of selected silicon compounds. The calculated ground‐state geometries and ionization potentials of the silicon hydrides: SiH, SiH2, SiH3and SiH4and of the ions SiH+, SiH−, SiH+3and SiH−3are in good agreement with the best theoretical and experimental results — whenever these are available. All the singly excited states of SiH2originating from a given occupied orbital are evaluated in a single calculation. For those few valence states for which reliable experimental and theoretical data exist, agreement is good. The conceptual and computational advantages of transforming the basis set of silicon into a set of atomic pseudoorbitals are demonstrated by repeating the latter computations with increasingly large sets of atomic orbitals, ranging from [3s, 3p] to [3s, 3p, 3d, 4s, 4p]. The utility of this procedure for reliable and economical calculations on organosilicon compounds is illustrated with a similar set of computations on the ground‐state properties and vertical excitation energies of Si(CH3)2. The effect of methyl substitution on the ground‐state properties of silylene and on the energetic ordering of its excited stat