Calculated properties of solidO2under pressure at low temperature

The zero-temperature properties of solid ${\mathrm{O}}_2$ are calculated at pressures 0≤P≤70 kbar by minimizing the Gibbs free energy at each pressure and then using the equilibrium lattice parameters in a harmonic-lattice-dynamics calculation of the dynamical properties. The minimization is based upon a continuously deformable monoclinic unit cell where the internal energy is composed of the static lattice energy and the properly weighted Einstein zero-point energies of the phonon, libron, and vibron modes. The lattice parameters, binding energy, compressibility, pressure-volume relation, libron, phonon, and vibron frequencies, phonon velocities, and dispersion curves are determined at each pressure. The lattice parameters and the phonon, libron, and vibron frequencies show clear evidence of a monoclinic to orthorhombic Fmmm structural phase transition at 24±1 kbar. A change in slope of these quantities is also observed at P≃7 kbar. A detailed comparison of these results with recent Raman scattering measurements is given.