  In this work, we analyze the evolution of the phase-space structures of KCN
molecular system as a function of the vibrational energy using Lagrangian
descriptors. For low energies, the motion is mostly regular around the absolute
minimum of the potential energy surface. As the energy increases, the phase
space combines regions with regular and chaotic motion, a difference that is
well captured by the Lagrangian descriptors. We show that the dynamics is
mostly governed by the invariant manifolds of the stretch periodic orbits
located at the top of one of the energetic barriers of the system. Furthermore,
we show a perfect agreement between the bifurcation theory and the differences
observed in the phase-space structures as the vibrational energy is modified.
The accuracy of our calculations is also assessed by explicit comparison with
the invariant manifolds computed using linear dynamics.
