  Chlorosomes are the largest and most efficient natural light-harvesting
antenna systems. They contain thousands of pigment molecules -
bacteriochlorophylls (BChls)- that are organized into supramolecular aggregates
and form a very efficient network for excitonic energy migration. Here, we
present a theoretical study of excitation energy transfer (EET) in the
chlorosome based on experimental evidence of the molecular assembly. Our model
for the exciton dynamics throughout the antenna combines a stochastic time
propagation of the excitonic wave function with molecular dynamics simulations
of supramolecular structure, and electronic structure calculations of the
excited states. The simulation results reveal a detailed picture of the EET in
the chlorosome. Coherent energy transfer is significant only for the first 50
fs after the initial excitation, and the wavelike motion of the exciton is
completely damped at 100 fs. Characteristic time constants of incoherent energy
transfer, subsequently, vary from 1 ps to several tens of ps. We assign the
time scales of the EET to specific physical processes by comparing our results
with the data obtained from time-resolved spectroscopy experiments.
