The role of discrete molecular modes in the coherent exciton dynamics in FMO

Abstract

We investigate the question of whether discrete molecular modes can protect quantum coherence in electronic exciton dynamics. We present numerically exact results for the electronic energy transfer dynamics in the Fenna–Matthews–Olson (FMO) molecular aggregate. In particular, we determine its single excitation subspace dynamics within an open quantum dynamics approach based on input parameters from an atomistic classical molecular dynamics modelling for FMO by Olbrich et al (2011a J. Chem. Phys. B 115 8609; 2011b J. Chem. Phys. Lett. 2 1771). The fluctuational spectra obtained there exhibit several discrete molecular vibrational modes and a surprisingly strong continuous background induced by the solvent. The latter alone causes overdamped excitonic dynamics, although its absolute strength might be somewhat overestimated. Nevertheless, it allows us to address the principal question of whether the discrete vibrational modes are strong enough to induce additional quantum coherent exciton dynamics which would be overdamped without them. We find that including them yields only negligible effects. By this, we can rule out a scenario of quantum coherence in the FMO exciton dynamics protected by the discrete molecular mode. Only when the dynamics is coherent already without the discrete modes, its coherence might be additionally enhanced.