  Under a pressure of ~150 GPa solid molecular hydrogen undergoes a phase
transition accompanied by a dramatic rise in infra-red 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 the molecules in the crystal, so that
the commonly used Clausius-Mossotti description in terms of polarizable,
non-overlapping molecular charge densities is inadequate already at low
pressures and even more so for the compressed solid.
