Modeling of extended solids using DFT and evolutionary algorithms.

We present overview of our recent results on simulation of poly-CO, mixtures of N2 and H2, and mixtures of N2 and CO gases in amorphous and crystalline phases under high pressure using density functional theory (DFT) and evolutionary algorithms. Structure of N-H extended network under high pressure was modelled using the evolutionary program USPEX based on plane wave DFT calculations with norm-conserving pseudopotentials. Range of the studied pressures was 10 -- 50 GPa on compression, and from 50 to 10 GPa on isotropic decompression of the extended network. Formation of an extended network with covalent bonds occurs between 30-50 GPa. Higher concentration of N requires higher pressure to form a covalent bond network. New structure of NH extended solids with high symmetry and covalent bonds are predicted: with C2M(C2H-3) symmetry group for 9:1 ratio, with PBAM (D2H$+$9) symmetry group for 4:1 ratio, and with P-1(CI-1) for 3:1 ratio of N$_{\mathrm{2}}$ to H$_{\mathrm{2}}$ gas. Modeling structure of N2-CO crystals using the same methods resulted in formation of crystals with covalent bonds and high symmetry: P41212 (D4$+$4) for 50{\%} of N2 and CO, CM (CS-3) for 80 N2 and 20{\%} CO, and R3 (C3-4) for 90{\%} N2 and 10{\%} CO at pressure of 50 GPa. We show that some of the structures, obtained at high pressures, may exist upon lowering of the pressures.