In their paper ‘Spiral chain O₄ form of dense oxygen’ (Proc. Natl. Acad. Sci. U.S.A., 2012, 109, 3, 751), L. Zhu and co-workers describe their predicted chain structure for oxygen. Under what conditions is this structure predicted to exist? Which other Group 16 elements form this structure? How do the predicted properties of this form of oxygen differ from those of other forms of the element?