  A finite (periodic) FPU chain is chosen as a convenient point for
investigating the energy exchange phenomenon in nonlinear oscillatory systems.
As we have recently shown, this phenomenon may occur as a consequence of the
resonant interaction between high-frequency nonlinear normal modes. This
interaction determines both the complete energy exchange between different
parts of the chain and the transition to energy localization in an excited
group of particles. In the paper, we demonstrate that this mechanism can exist
in realistic (asymmetric) models of atomic or molecular oscillatory chains.
Also, we study the resonant interaction of conjugated nonlinear normal modes
and prove a possibility of linearization of the equations of motion.
  The theoretical constructions developed in this paper are based on the
concepts of "effective particles" and Limiting Phase Trajectories. In
particular, an analytical description of energy exchange between the "effective
particles" in the terms of non-smooth functions is presented. The analytical
results are confirmed with numerical simulations.
