Predicting the crystal structure of [Formula: see text] high energy density material using ab initio evolutionary algorithms.

[Formula: see text] is the first successfully synthesized salt that has a polymeric nitrogen moeity ([Formula: see text]). Although 12 other [Formula: see text] salts followed, with [Formula: see text] and [Formula: see text] being the most stable, the crystal structure of [Formula: see text] remains unknown. Currently, it is impossible to experimentally determine the structures of [Formula: see text] due to its marginal stability and explosive nature. Here, following an ab initio evolutionary prediction and using only the stoichiometry of [Formula: see text] as a starting point, we were able to reveal the crystal structure of this high energy density material (HEDM). The [Formula: see text] symmetry of the [Formula: see text] cation, as suggested from earlier investigations, is confirmed to be the symmetry adopted by this polymeric nitrogen within the crystal. This result gave full confidence in the validity of this crystal prediction approach. While stability of the [Formula: see text] within the crystal is found to be driven by electronic considerations, the marginal stability of this HEDM is found to be related to a partial softening of its phonon modes.