  The recently proposed Einstein molecule approach is extended to compute the
free energy of molecular solids. This method is a variant of the Einstein
crystal method of Frenkel and Ladd[J. Chem. Phys. 81,3188 (1984)]. In order to
show its applicability, we have computed the free energy of a hard-dumbbells
solid, of two recently discovered solid phases of water, namely, ice XIII and
ice XIV, where the interactions between water molecules are described by the
rigid non-polarizable TIP4P/2005 model potential, and of several solid phases
that are thermodynamically stable for an anisotropic patchy model with
octahedral symmetry which mimics proteins.Our calculations show that both the
Einstein crystal method and the Einstein molecule approach yield the same
results within statistical uncertainty.In addition, we have studied in detail
some subtle issues concerning the calculation of the free energy of molecular
solids. First, for solids with non-cubic symmetry, we have studied the effect
of the shape of the simulation box on the free energy. Our results show that
the equilibrium shape of the simulation box must be used to compute the free
energy in order to avoid the appearance of artificial stress in the system that
will result in an increase of the free energy. In complex solids, such as the
solid phases of water, another difficulty is related to the choice of the
reference structure. As in some cases there is not an obvious orientation of
the molecules, it is not clear how to generate the reference structure. Our
results will show that,as long as the structure is not too far from the
equilibrium structure,the calculated free energy is invariant to the reference
structure used in the free energy calculations. Finally, the strong size
dependence of the free energy of solids is also studied.
