Dipole-quadrupole interactions and the nature of phase III of compressed hydrogen

A new class of strongly infrared (IR) active structures is identified for p hase III of compressed molecular H-2 by constant-pressure ab initio molecul ar dynamics and density-functional perturbation calculations. These are pla nar quadrupolar structures obtained as a distortion of low-pressure quadrup olar phases, after they become unstable at about 150 GPa due to a zone boun dary soft phonon. The nature of the II-III transition and the origin of the IR activity are rationalized by means of simple electrostatics, as the ons et of a stabilizing dipole-quadrapole interaction.