  Calculations of the optical properties of GeTe in the cubic NaCl and
rhombohedral ferroelectric structures are reported. The rhombohedral
ferroelectric distortion increases the band gap from 0.11 eV to 0.38 eV.
Remarkably, substantial changes in optical properties are found even at high
energies up to 5 eV. The results are discussed in relation to the bonding of
GeTe and to phase change materials based on it.
