FIG.1. The structure of (a) ice Ih and (b) ice Ic. Note the tetrahedral coordination and the openess of the structure. On each hydrogen bond, shown by a rod joining the oxygen atoms, lies one proton in an asymmetric position (not shown).Bond lengths, 275 pm, are indicated. The hexagonal c axis is labeled 732 pm, and one of the hexagonal a axes is labeled 450 pm. In (b) the cubic unit cell is outlined with dashed lines; dimensions are in pm at 110 K.