  The Statistical Multifragmentation Model is modified to incorporate the
Helmholtz free energies calculated in the finite temperature Thomas-Fermi
approximation using Skyrme effective interactions. In this formulation, the
density of the fragments at the freeze-out configuration corresponds to the
equilibrium value obtained in the Thomas-Fermi approximation at the given
temperature. The behavior of the nuclear caloric curve at constant volume is
investigated in the micro-canonical ensemble and a plateau is observed for
excitation energies between 8 and 10 MeV per nucleon. A kink in the caloric
curve is found at the onset of this gas transition, indicating the existence of
a small excitation energy region with negative heat capacity. In contrast to
previous statistical calculations, this situation takes place even in this case
in which the system is constrained to fixed volume. The observed phase
transition takes place at approximately constant entropy. The charge
distribution and other observables also turn out to be sensitive to the
treatment employed in the calculation of the free energies and the fragments'
volumes at finite temperature, specially at high excitation energies. The
isotopic distribution is also affected by this treatment, which suggests that
this prescription may help to obtain information on the nuclear equation of
state.
