We illustrate a systematic search method for determining hydrogen atomic arrangements in the hydrogen storage material, ${\mathrm{Li}}_2\mathrm{N}\mathrm{H}$. Our method relies on total-energy density-functional calculations, and yields a minimum-energy crystal structure for ${\mathrm{Li}}_2\mathrm{N}\mathrm{H}$ as well as several low-energy metastable structures. Linear-response calculated Born effective charges show strong ionic interactions between the Li and N-H dimers, while the bonding between the N and H has covalent character. Including vibrational contributions, our orthorhombic $Pnma$ structure yields a hydrogen storage ${\mathrm{Li}}_2\mathrm{N}\mathrm{H}∕\mathrm{Li}\mathrm{N}{\mathrm{H}}_2$ reaction enthalpy of $63.7\mathrm{kJ}∕\mathrm{mol}{\mathrm{H}}_2$ at $T=0\mathrm{K}$, and $74.8\mathrm{kJ}∕\mathrm{mol}{\mathrm{H}}_2$ at $T=300\mathrm{K}$, in good agreement with experimental reports of $\sim 66\mathrm{kJ}∕\mathrm{mol}{\mathrm{H}}_2$ for this reaction.

• Received 11 January 2006

DOI:https://doi.org/10.1103/PhysRevB.73.220101