  Site-occupancy disorder, defined as the non-periodic occupation of lattice
sites in a crystal structure, is a ubiquitous phenomenon in solid-state physics
and chemistry. Examples are mineral solid solutions, synthetic
non-stoichiometric compounds and metal alloys. The experimental investigation
of these materials using diffraction techniques only provides averaged
information of their structure. However, many properties of interest in these
solids are determined by the local geometry and degree of disorder, which
escape an "average crystal" description, either from experiments or from
theory. In this paper, we describe a methodology for the computer simulation of
site-disordered solids, based on the consideration of configurational ensembles
and statistical mechanics, where the number of occupancy configurations is
reduced by taking advantage of the crystal symmetry of the lattice.
Thermodynamics and non-thermodynamic properties are then defined from the
statistics in the symmetry-adapted configurational ensemble. We will briefly
summarize and discuss some recent applications of this methodology to problems
in materials science.
