We review two well-known definitions present in the literature. which are used to define the heat or energy flux in one dimensional chains. One definition equates the energy variation per particle to a discretized flux difference. which we here show it also corresponds to the flux of energy in the zero wavenumber limit in Fourier space. https://www.ealisboa.com/flash-pick-OO9406-Transparent-Prizm-Sapphire-XL-deal-zone/