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