Phonon occupations fluctuate since the distribution of thermal energy across all of the oscillators in a solid changes with time, but the average phonon occupation is determined by Bose-Einstein statistics. The complex movement of atoms (and their total kinetic energy) is thus the sum over all phonon modes and their instantaneous occupation. It follows, then, that a phonon-based definition of 𝐸 A should have both a frequency dependence and phonon occupation dependence.