AtomisticModeling of Two-Dimensional Electronic Spectraand Excited-State Dynamics for a Light Harvesting 2 Complex.

The Light Harvesting 2 (LH2) complexis a vital part of the photosystemof purple bacteria. It is responsible for the absorption of lightand transport of the resulting excitations to the reaction centerin a highly efficient manner. A general description of the chromophoresand the interaction with their local environment is crucial to understandthis highly efficient energy transport. Here we include this interactionin an atomistic way using mixed quantum-classical (molecular dynamics)simulations of spectra. In particular, we present the first atomisticsimulation of nonlinear optical spectra for LH2 and use it to studythe energy transport within the complex. We show that the frequencydistributions of the pigments strongly depend on their positions withrespect to the protein scaffold and dynamics of their local environment.Furthermore, we show that although the pigments are closely packedthe transition frequencies of neighboring pigments are essentiallyuncorrelated. We present the simulated linear absorption spectra forthe LH2 complex and provide a detailed explanation of the states responsiblefor the observed two-band structure. Finally, we discuss the energytransfer within the complex by analyzing population transfer calculationsand 2D spectra for different waiting times. We conclude that the energytransfer from the B800 ring to the B850 ring is mediated by intermediatestates that are delocalized over both rings, allowing for a stepwisedownhill energy transport. [ABSTRACT FROM AUTHOR]