  A generalized master equation (GME) governing quantum evolution of modular
exciton density (MED) is derived for large scale light harvesting systems
composed of weakly interacting modules of multiple chromophores. The GME-MED
offers a practical framework to incorporate real time coherent quantum dynamics
calculations at small length scales into dynamics over large length scales,
without assumptions of time scale separation or specific forms of intra-module
quantum dynamics. A test of the GME-MED for four sites of the
Fenna-Matthews-Olson complex demonstrates how coherent dynamics of excitonic
populations over many coupled chromophores can be accurately described by
transitions between subgroups (modules) of delocalized excitons.
