  Non-Markovian and non-equilibrium phonon effects are believed to be key
ingredients in the energy transfer in photosynthetic complexes, especially in
complexes which exhibit a regime of intermediate exciton-phonon coupling. In
this work, we utilize a recently-developed measure for non-Markovianity to
elucidate the exciton-phonon dynamics in terms of the information flow between
electronic and vibrational degrees of freedom. We study the measure in the
hierarchical equation of motion approach which captures strong system-bath
coupling effects and non-equilibrium molecular reorganization. We propose an
additional trace-distance measure for the information flow that could be
extended to other master equations. We find that for a model dimer system and
the Fenna-Matthews-Olson complex that non-Markovianity is significant under
physiological conditions.
