Why quantum coherence is not important in the Fenna-Matthews-Olsen Complex

TL;DR

This paper introduces an improved computational method for solving the Hierarchical Equations of Motion, enabling exact calculations of energy transfer in the FMO complex and showing that quantum coherence does not significantly enhance photosynthesis efficiency.

Contribution

An improved HEOM technique that reduces memory cost by over 75%, allowing for exact, large-scale simulations of the FMO complex and challenging the presumed importance of quantum coherence in photosynthesis.

Findings

Exact 24-site FMO results show coherence is not crucial for efficiency.

Incoherent Forster theory overestimates energy transfer rates.

Quantum coherence does not significantly improve photosynthesis efficiency.

Abstract

We develop and present an improvement to the conventional technique for solving the Hierarchical Equations of Motion which reduces the memory cost by more than 75% while retaining the same convergence rate and accuracy. This allows for a full calculation of the population dynamics of the 24-site FMO trimer for long timescales with very little effort, and we present the first fully converged, exact results for the 7-site subsystem of the monomer, and for the full 24-site trimer. Owing to this new approach, our numerically exact 24-site, 2-exponential results are the most demanding HEOM calculations performed to date, surpassing the 50-site, 1-exponential results of Strumpfer and Schulten [2012, J. Chem. Thy. & Comp., 8, 2808]. We then show where our exact 7-site results deviate from the approximation of Ishizaki and Fleming [2009, Proc. Natl. Acad. Sci. USA, 106, 17255]. Our exact results are then compared to calculations using the incoherent Forster theory, and it is found that the energy transfer from the antenna to the reaction center occurs more than 50 times faster than the fluorescence lifetime of the excitation, whether or not coherence is considered. This means that coherence is not likely to improve the efficiency of the photosynthesis. In fact, the incoherent theory often tends to over-predict the rates of energy transfer, suggesting that in some cases electronic coherence may actually slow down the photosynthetic process.