Quantum Dynamics in Large Systems: Exploring the Complex Pathways of Photosynthesis

Until recently, simulations of time evolution in systems of many atoms have relied either entirely on classical dynamics or approximate ways of including quantum mechanical effects. This lecture will present emerging path integral and semiclassical approaches and their applications to the quantum dynamics of energy and electron transfer in bacterial photosysnthesis.

Using Feynman's path integral representation of quantum mechanics, ensemble-averaged quantities (e.g. correlation functions or the reduced density matrix) can be expressed in terms of a reduced dimension integral, where the effects of the environment on the degrees of freedom of interest enter via an influence functional. If the environment consists of harmonic phonon-like modes, the influence functional is available analytically, and recent work has introduced efficient semiclassical methods based on forward-backward propagation that can be used in solvents with anharmonic potentials. Due to dephasing effects and/or thermal fluctuations, memory interactions arising from the environment have a finite span. Exploiting that fact leads to an iterative procedure for evaluating the path integral on a minimal storage grid generated from a system-specific discrete variable representation. This scheme circumvents the sign problem, allowing accurate calculation of the dynamics over long time intervals.

The photosynthetic apparatus of purple bacteria consists of ring-shaped light harvesting complexes that absorb visible light and funnel the energy to the reaction center, initiating a series of electron transfer reactions. Employing the path integral representation of quantum statistical mechanics we have investigated the process of energy transfer within light harvesting systems. Although perfect coherence is in principle possible at zero temperature in the absence of any disorder, our work has shown that under physiological conditions the energy transfer proceeds primarily via a hopping mechanism and thus the coherence length is very short. The initial 3 ps of charge separation have been controversial. We have demonstrated that the available kinetic data on wild-type and substituted reaction centers are compatible with a two-step mechanism that utilizes a chlorophyll monomer as a bridge.

Speaker: Nancy Makri, Departments of Chemistry and Physics, University of Illinois, Urbana-Champaign, USA
Time: Wednesday, 10 January 2001, 13:00