Stable and metastable states and the formation and destruction of breathers in the discrete nonlinear Schrِdinger equation

Statistical mechanics explains many localization phenomena of lattices such as the discrete nonlinear Schrِdinger equation. However, numerical simulations show that the complete thermalization is rarely achieved. Instead, one observes metastable statistical states that are robust when excited locally. This paper investigates thermodynamically metastable states where the trajectory is confined to some part of the energy shell. The partition function and the entropy are computed with a perturbation method. This method is applicable to stable and metastable states, and it allows us to give approximative analytic expressions for the entropy in the complete thermodynamic state space.