Improved Variational Master Equation Theory for the Excitation Energy Transfer

Abstract

In photosynthetic light harvesting systems, the strength of the excitation energy transfer (EET) interaction between pigments and the strength of the exciton–phonon coupling are of a comparable magnitude. Established theories are able to reproduce the EET processes for limiting cases, but the intermediate case has proven to be more difficult. We present here an improvement of the quantum master equation theory based on the variational principle to adequately describe the EET under intermediate conditions. The modified variational parameter is determined by a free-energy minimization based on the second Bogoliubov inequality. We show that the perturbation term given by our modified theory leads to a reorganization energy dependence of the EET rate that is closer to that determined by the hierarchical equation of motion.