Oxygen at high pressures: a theoretical approach to monoatomic phases.

We have studied the ζ-phase of solid oxygen using the generalized gradient approximation in the density functional approach. Calculations of total energies and pressures have been carried out for the prototype of diatomic ζ-phase and other hypothetical monoatomic crystal structures. The diatomic phase was found to be stable over a wide range of pressure (100-2000 GPa). The stacking of molecular layers is discussed in comparison with the available experimental data.