A density functional theory investigation of the recently identified hydrogen storage reaction LiNH₂ + LiH ↔ Li₂NH + H₂ is described. The electronic structure and enthalpy of formation ΔH of each constituent are calculated in both the generalized gradient approximation (GGA) and the local density approximation (LDA). Zero point energies and finite temperature corrections to ΔH are derived via calculation of the vibrational spectra. We find that the GGA provides much more favorable agreement with experiment than the LDA for the structural parameters and for ΔH(LiNH), ΔH(LiH), and the overall reaction enthalpy.