Magnetism and vibrations in the phase $\epsilon$ of Oxygen

Sandwiched between a set of magnetic phases at lower pressure, and a non magnetic phase at higher pressure, the magnetic state of phase $\epsilon$ of oxygen has so far been elusive, together with its crystal structure. Neutron diffraction data indicate absence of antiferromagnetism, but do not exclude a ferromagnetic order. The recent refinement of the internal atomic positions from single-crystal diffraction finally provides us with a correct structural model to study the possible occurrence of a magnetic ground state. By employing non-collinear spin-polarized density-functional theory we show that the ground state of $\epsilon$-O$_2$ is non magnetic. We also calculate vibrational spectra and show that $\epsilon$-O$_2$ possesses an additional vibron mode with large Raman cross section, not seen in experiments yet.