
56 Perturbation Theories for Fluids and Solids
molecular configuration, and the average is performed over all possible positions and
all possible molecular configurations.
Most of the aforementioned methods are unsuitable to obtain the chemical poten-
tial of a crystalline solid. However, procedures based on the coupling between the
actual potential energy of the system and that of a reference system, similar to those
in Equations 3.76 and 3.77, are useful (Frenkel and Ladd 1984, Frenkel and Smit
2002). In this case, the reference system is the Einstein crystal, whose thermodynamic
properties are known, with the same structure as the solid under consideration. ...