The solid QMC calculations reported on in chapters to all involve either germanium or silicon in the diamond structure. This is the stable structure for both materials at room temperature and pressure. The diamond structure consists of a face centred cubic (FCC) Bravais lattice with a two atom basis. The conventional unit cell is shown in figure and contains four lattice sites and eight atoms. The tetrahedral bonding between atoms in the diamond structure can be clearly seen and is characteristic of the directional covalent bonding found in Group IV of the periodic table. The lattice constant of germanium is 5.65Å and that of silicon is 5.29Å. The diamond structure is relatively ``empty'', with a maximum of 34% of the available volume being filled by touching hard spheres. The primitive unit cell is shown in figure . Each primitive cell contains a basis of two atoms.
Figure: Conventional Unit Cell of the diamond structure. One of the atoms in the basis is shown in red and the other in blue.
Figure: Primitive unit cell of the FCC Bravais Lattice