  We propose a first principle computation of the thermodynamics of simple
fragile glasses starting from the two body interatomic potential.
  A replica formulation translates this problem into that of a gas of
interacting molecules, each molecule being built of $m$ atoms, and having a
gyration radius (related to the cage size) which vanishes at zero temperature.
We use a small cage expansion, valid at low temperatures, which allows to
compute the cage size, the specific heat (which follows the Dulong and Petit
law), and the configurational entropy.
