Density‐functional theory for solid nitrogen and carbon dioxide at high pressure

Structural properties of solid N₂ and CO₂ under pressure have been studied with a recently reported theory. The model allows for calculation of the structure and dissociation energy of molecular crystals, using no empirical parameters where dispersion energy is included as a sum of the pair interactions. Comparison with available low temperature experimental results shows good agreement with average errors in lattice constants of about 1% and in lattice energies of about 6%. Calculations were performed on nitrogen in a number of structures, including the experimentally observed Pa3 (α), P4₂/mnm (γ), P6₃/mmc (β), and Pm3n structures, as well as a recently proposed structure with space group R3̄m. The R3̄m structure was found to be more stable than the Pa3 structure at about 10 kbar. Calculations were carried out on carbon dioxide in the Pa3 structure. Comparison of the calculated pressure‐volume curves with room temperature experimental curves to 100 kbar shows very close agreement for both N₂ and CO₂.