A first-principles approach is used to study the structural and dynamical stability of the hypothetical RbN and CsN compounds. First, I review briefly our earlier work on the origin of half-metallicity and then I present detailed results on the mechanical and dynamical properties obtained from the ultra soft pseudopotential approach. The elastic constants of both compounds in their equilibrium B2 phase confirm the mechanical stability. The phonon dispersions are calculated from the density functional perturbation theory and show no imaginary frequencies. Other thermodynamic properties are also obtained from the quasi-harmonic approximation and show good behaviour with temperature.