Low-energy linear structures in dense oxygen: implications for the epsilon phase.

Using density functional theory implemented within the generalized gradient approximation, a new nonmagnetic insulating ground state of solid oxygen is proposed and found to be energetically favored at pressures corresponding to the epsilon phase. The newly predicted static ground state is composed of linear herringbone-type chains of O2 molecules and has Cmcm symmetry (with an alternative monoclinic cell). Importantly, this phase supports IR-active zone-center phonons, and their computed frequencies are found to be in broad agreement with recent infrared absorption experiments.