The discrete lattice does, however, influence the kink motion in a unique way with the existence of the Peierls–Nabarro (PN) potential V PN ( X ) , where X is the position of the kink's center. The existence of the PN potential is due to the lack of translational invariance in a discrete chain. In the continuum limit the system is invariant for any translation of the kink along the chain. For a discrete chain, only those translations that are an integer multiple of the lattice spacing a s leave the system invariant. The PN barrier, E PN , is the smallest energy barrier for a kink to overcome so that it can move through the lattice. The value of the PN barrier is the difference between the kink's potential energy for a stable and unstable stationary configuration. The stationary configurations are shown schematically in the figure.