  We report systematic first principles density functional study on the
electronic structure, elastic and optical properties of nitrogen based solid
hydrogen storage materials LiNH$_2$, NaNH$_2$, KNH$_2$, and RbNH$_2$. The
ground state structural properties are calculated by using standard density
functional theory and also dispersion corrected density functional theory. We
find that van der Waals interactions are dominant in LiNH$_2$ whereas they are
relatively weak in other alkali metal amides. The calculated elastic constants
show that all the compounds are mechanically stable and LiNH$_2$ is found to be
stiffer material among the alkali metal amides. The melting temperatures are
calculated and which follows the order RbNH$_2$ $<$ KNH$_2$ $<$ NaNH$_2$ $<$
LiNH$_2$. The electronic band structure is calculated by using the Tran-Blaha
modified Becke-Johnson potential and found that all the compounds are
insulators with a considerable band gap. The [NH$_2$]$^-$ derived states are
completely dominating in the entire valence band region while the metal atom
states occupy the conduction band. The calculated band structure is used to
analyze the different interband optical transitions occur between valence and
conduction bands. Our calculations show that these materials have considerable
optical anisotropy.
