  The free energy of the solid and fluid phases of the Girifalco C60 model are
determined through extensive Monte Carlo simulations. In this model the
molecules interact through a spherical pair potential, characterized by a
narrow and attractive well, adjacent to a harshly repulsive core. We have used
the Widom test particle method and a mapping from an Einstein crystal, in order
to estimate the absolute free energy in the fluid and solid phases,
respectively; we have then determined the free energy along several isotherms,
and the whole phase diagram, by means of standard thermodynamic integrations.
  We highlight how the interplay between the liquid-vapor and the liquid-solid
coexistence conditions determines the existence of a narrow liquid pocket in
the phase diagram, whose stability is assessed and confirmed in agreement with
previous studies. In particular, the critical temperature follows closely an
extended corresponding-states rule recently outlined by Noro and Frenkel [J.
Chem. Phys. 113:2941 (2000)].
  We discuss the emerging "energetic" properties of the system, which drive the
phase behavior in systems interacting through short-range forces [A. A. Louis,
Phil. Trans. R. Soc. A 359:939 (2001)], in order to explain the discrepancy
between the predictions of several structural indicators and the results of
full free energy calculations, to locate the fluid phase boundaries.
  More generally, we aim to provide extended reference data for calculations of
the free energy of the C60 fullerite in the low temperature regime, as for the
determination of the phase diagram of higher order fullerenes and other
fullerene-related materials, whose description is based on the same model
adopted in this work.
