The Self-Trapping Line of the Holstein Molecular Crystal Model in One   Dimension
  The ground state of the Holstein molecular crystal model in one dimension is studied using the Global-Local variational method, analyzing in particular the total energy, kinetic energy, phonon energy, and interaction energy over a broad region of the polaron parameter space. Through the application of objective criteria, a unique curve is identified that simply, accurately, and robustly locates the self-trapping transition separating small polaron and large polaron behavior. 
A. H. Romero, David W. Brown, and Katja Lindenberg