Atomistic Modeling of Two-Dimensional Electronic Spectra and Excited State Dynamics for a Light Harvesting 2 Complex

The Light Harvesting 2 (LH2) complex is a vital part of the photosystem of purple bacteria. It is responsible for the absorption of light and transport of the resulting excitations to the reaction center in a highly efficient manner. In order to understand this highly efficient energy transport a general description of the chromophores and the interaction with their local environment is crucial. Here, we include this interaction in a full-atomistic way using mixed quantum-classical(molecular dynamics) simulations of spectra. In particular, we present the first full-atomistic simulation of non-linear optical spectra for LH2 and use it to study the energy transport within the complex. We show that the frequency distributions of the pigments strongly depend on their positions with respect to the protein scaffold and dynamics of their local environment. Furthermore, we show that although the pigments are closely packed, the transition frequencies of neighboring pigments are essentially uncorrelated. We present the simulated linear absorption spectra for the LH2 complex and provide a detailed explanation of the states responsible for the observed two-band structure. Finally, we discuss the energy transfer within the complex by analyzing population transfer calculations and two-dimensional spectra for different waiting times. We conclude that the nature of the eigenstates allows for a fast downhill energy transfer from the B800 ring to the B850 ring.