 This study investigates under what circumstances Forster theory of electronic resonance energy transfer breaks down in molecular aggregates by simulating exciton diffusion on the femtosecond timescale using the Louisville von Neumann equation of motion. The focus is on spatial and temporal deviations between exciton dynamics driven by electronic couplings calculated from Forster theory and those calculated from quantum electrodynamics QED. The results indicate that Forster coupling is valid when dipole centres are within a few nanometres of one another, but as distance increases, intermediate and far-zone coupling terms play non-negligible roles and Forster theory begins to break down. These findings could have implications for spectroscopic ruler techniques and energy harvesting materials design principles. This article was offered by James E. Frost and Garth A. Jones.