This Demonstration shows the propagation of an energy pulse in a 1D harmonic chain with 3D vibrations, to study the influence of the lattice constant on heat conduction. Starting from a uniform initial temperature and adding a pulse of energy to the central atom at position , the snapshots of are the averages over 1000 observations taken at times . A large lattice constant () behaves approximately as a 1D harmonic chain while a small lattice constant () brings out the influence of 3D vibrations. The contrast between the two cases depends on the magnitude of the lattice constant.