Analytical and numerical calculations for a reduced Fermi-Pasta-Ulam chain demonstrate that energy localization does not require more than one conserved quantity. Clear evidence for the existence of a sharp delocalization-localization transition at a critical amplitude Ac is given. Approaching Ac from above and below, diverging time scales occur. Above Ac, the energy packet converges towards a discrete breather. Nevertheless, ballistic energy transportation is present, demonstrating that its existence does not necessarily imply delocalization.PACS numbers: 05.60Cd, 63.20.Pw One of the classical investigations of relaxation dynamics of macroscopic systems is to determine the time evolution of a perturbed equilibrium state. If this initial state converges to equilibrium the system is called mixing, implying ergodicity, and it is nonmixing, otherwise. An important question is: Does there exist a sharp ergodicity breaking transition under variation of a physical control parameter like temperature or strength of perturbation?Within a mode coupling theory for supercooled liquids such a dynamical glass transition has been found, but its sharpness seems to result from the mode coupling approximations (for reviews see [1,2]). It is not our purpose to contribute to the theory of glass transition, but to study the influence of anharmonicity on the relaxational behavior at zero-temperature. In that case the generic lowest energy state of a particle system is a crystal. One may ask a similar question as above: Does an initially localized energy excitation spread over the complete crystal, or not? In case of small excitation amplitudes, one can use the harmonic approximation. Then the time evolution of an initial configuration can be determined, exactly [3]. For one-dimensional harmonic lattices the results are particularly simple [4,5]. Independent of the strength and size of the excitation it always spreads over the full system, and energy transportation is ballistic, provided that there is no disorder. That infinite harmonic crystals are ergodic in general, has been proven rigorously [6]. If the excitation amplitude increases, anharmonicity gets important.Let us neglect any disorder, but taking anharmonic interactions into account. Discreteness of the lattice combined with anharmonicity allows for the existence of localized periodic vibrations, called discrete breathers (DB). For reviews see Ref. [7]. Their existence suggests that under certain conditions a localized excitation may converge to a DB, whereby suppressing complete energy spreading. Indeed, numerical solutions of the discrete nonlinear Schrödinger equation (DNLS) [8] and references wherein, the Klein Gordon chain (KG) [9] and the β-Fermi-Pasta-Ulam chain (FPU) [10,11] demonstrate generation of DB and their role for slow energy relaxation. Particularly, the numerical results in Refs. [8,11] give evidence that DB generation from an initially localized excitation requires an excitation amplitude which is large enough. This has been supported by analytical stu...