Polarization and Strong Infrared Activity in Compressed Solid Hydrogen

Under a pressure of ≃150GPa solid molecular hydrogen undergoes a phase transition accompanied by a dramatic rise in infrared absorption in the vibron frequency range. We use the Berry's phase approach to calculate the electric polarization in several candidate structures finding large, anisotropic dynamic charges and strongly IR-active vibron modes. The polarization is shown to be greatly affected by the overlap between molecules in the crystal, so that the commonly used Clausius-Mossotti description in terms of polarizable, nonoverlapping molecular charge densities is inadequate already at low pressures and even more so for the compressed solid.