  By means of ab initio metadynamics runs we explored the lower-pressure region
of the phase diagram of germanium. A monoclinic germanium phase with
four-membered rings, less dense than diamond and compressible into \beta-tin
phase (tI4) was found. A metallic bct-5 phase, mechanically stable down to room
conditions appeared between diamond and tI4. mC16 is a narrow-gap
semiconductor, while bct-5 is metallic and potentially still superconducting in
the very low pressure range. This finding may help resolving outstanding
experimental issues.
