A time-dependent modified Redfield theory for absorption spectra applied to light-harvesting systems

A time-dependent modified Redfield theory is used to calculate the linear absorption spectrum for an ensemble of B850 rings of the light-harvesting system LH2 of purple bacteria. This theory is an extension of the standard modified Redfield theory without invoking the Markov approximation. The results for the modified Redfield theory with and without Markov approximation are compared to the spectra obtained from a time-local second-order perturbation theory in the system-bath coupling sometimes also called time-dependent Redfield theory. The influence of the shape of the spectral density on the linear absorption spectrum is demonstrated for single samples and in the ensemble average. Non-Markovian effects clearly show up in the high-energy part of the static absorption line shapes. Among the different spectral densities studied is one of a purple bacterium obtained earlier by a molecular dynamics simulation.