In connection with the recent photoelectron spectroscopy of negatively charged Na atom in ammonia clusters, the geometries, electronic state, vertical detachment energies, and harmonic frequencies of [Na(NH3)(n)](-)-(n =1-4) have been studied by the ab initio MO method. Structures having as many Na-N bonds as possible becomes more stable than other isomers as it grows. The Na 3 s electrons are widely sp;ead and delocalized in space outside the [Na(NH3)(n)](+) core for n greater than or equal to2. The dramatic redshifts of the photoelectron band for the 3(2)P-type transition with inreasing n reflect the electronic change from an atomic state to one-center Rydberg-type states in the neutrals. The frequencies of the combined vibrations of the NH3 mode are nearly degenerate and are expected to coalesce into only one strong IR band in the-NH stretch region irrespective of n. (C): 2000 American Institute of Physics. [S0021-9606(00)0945-X].