  We present a detailed analysis of the modulational instability of the
zone-boundary mode for one and higher-dimensional Fermi-Pasta-Ulam (FPU)
lattices. Following this instability, a process of relaxation to equipartition
takes place, which we have called the Anti-FPU problem because the energy is
initially fed into the highest frequency part of the spectrum, at variance with
the original FPU problem (low frequency excitations of the lattice). This
process leads to the formation of chaotic breathers in both one and two
dimensions. Finally, the system relaxes to energy equipartition on time scales
which increase as the energy density is decreased. We show that breathers
formed when cooling the lattice at the edges, starting from a random initial
state, bear strong qualitative similarities with chaotic breathers.
