Exciton-phonon dynamics on complex networks: Comparison between a perturbative approach and exact calculations

A method combining perturbation theory with a simplifying ansatz is used to describe the exciton-phonon dynamics in complex networks. This method, called PT*, is compared to exact calculations based on the numerical diagonalization of the exciton-phonon Hamiltonian for eight small-sized networks. It is shown that the accuracy of PT* depends on the nature of the network, and three different situations were identified. For most graphs, PT* yields a very accurate description of the dynamics. By contrast, for the Wheel graph and the Apollonian network, PT* reproduces the dynamics only when the exciton occupies a specific initial state. Finally, for the complete graph, PT* breaks down. These different behaviors originate in the interplay between the degenerate nature of the excitonic energy spectrum and the strength of the exciton-phonon interaction so that a criterion is established to determine whether or not PT* is relevant. When it succeeds, our study shows the undeniable advantage of PT* in that it allows us to perform very fast simulations when compared to exact calculations that are restricted to small-sized networks.