  The effective potential formalism is applied to glass forming liquids,
choosing a coupling potential such that the "order parameter", conjugated to
the coupling strength, is the mean square displacement of the particles from
their position in the quenched reference configuration. The potential is linear
in some interval of its argument, signaling the coexistence of two phases in
the system, one with low and one with high mean square displacement. Within
this formalism, one can also compute the free energy of metastable states and
their complexity.
