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Thermodynamically stable phases of sodium amide (NaNH2) at pressures up to 20 GPa have been determined using the ab initio evolutionary structure prediction. We find that the ground-state phase alpha-NaNH2 (orthorhombic, Fddd) first transforms into beta-NaNH2 (orthorhombic, P2(1)2(1)2) at 2.2 GPa; then, gamma-NaNH2 (monoclinic, C2/c) becomes stable at 9.4 GPa. In addition to strong ionic bonding between Na+ and [NH2](-) ions and covalent bonding between H and N in NH2 groups, the N- H center dot center dot center dot N hydrogen bonding between neighboring NH2 groups could not be ignored anymore in the high-pressure beta-NaNH2, as suggested by the analysis of charge density distribution and structural and vibrational properties. The covalent N H bonds in the high-pressure phase of NaNH2 are weakened by additional hydrogen bonding, which could be favorable for the hydrogen desorption