  We present a statistical mechanics description to study the ground state of
quantum systems. In this approach, averages for the complete system are
calculated over the non-interacting energy levels. Taking different interaction
parameter, the particles of the system fall into non-interacting microstates,
corresponding to different occupation probabilities for these energy levels.
Using this novel thermodynamic interpretation we study the Hubbard model for
the case of two electrons in two sites and for the half-filled band on a
one-dimensional lattice. We show that the form of the entropy depends on the
specific system considered.
