Fig. 1: Final and competing structures during the compression of CO + O2 mixtures.
Results from isothermal compression of CO + O2 mixtures using NVT-AIMD simulations along the T = 300 K isotherm. a Polymerized amorphous product in an 864-atom cell at 32.6 ± 0.7 GPa and ρ = 3.23 g/cc after simulation time of 11 ps at 300 K. Red and brown spheres indicate oxygen and carbon atoms, respectively, with chain and ring regions color-coded as shown. b Comparison of simulation results obtained with different cell sizes at ρ = 3.23 g/cc. Shown are the percentage of atoms locked in −O−C− and −C−C− bonds, and the equilibrated pressure P. Convergence is reached with 432-atom simulations. Violet and gray spheres correspond to oxygen and carbon atoms, respectively. The dimensions of the cubic box simulation cells are shown in Angstroms. c Starting from a molecular gaseous mixture of CO and O2, labeled as phase mg, the resulting phases from AIMD and metadynamics searches are shown in perspective view. The polymeric crystalline phases pc1 (84-atom Pbcn) and pc2 (36-atom P1) were obtained from constrained metadynamics searches. At pressures above 10 GPa, the polymeric amorphous phase pa is energetically preferred, as can be seen in the enthalpy versus pressure and density plots. The brown arrows and markers point to the change in the enthalpy and density when NPT-AIMD simulations are performed on the crystalline phases.
