  In this note we explain the use of the cavity method directly at zero
temperature, in the case of the spin glass on a Bethe lattice. The computation
is done explicitly in the formalism equivalent to 'one step replica symmetry
breaking'; we compute the energy of the global ground state, as well as the
complexity of equilibrium states at a given energy. Full results are presented
for a Bethe lattice with connectivity equal to three.
