The potential energy surfaces of ~- and P~- were investigated using ab initio electronic structure calculations. Additivity of correlation and basis set effects was used to estimate relative energies. The tetrahedral structures of N~- and P~- are predicted to be minima on the respective potential energy surfaces. Ab initio classical trajectory calculations were carried out in order to elucidate possible dissociation paths of tetrahedral ions. The dissociation barrier was estimated to be 32.5 kcaVmol for N~- and 5.5 kcaVmol for p~-. Ionization potentials for the tetrahedral structures of N~- and p~- were calculated to be 0.39 and 0.32 eV, respectively.