Modeling Electronic-Nuclear Interactions for Excitation Energy Transfer Processes in Light-Harvesting Complexes

An
accurate approach for computing intermolecular and intrachromophore
contributions to spectral densities to describe the electronic–nuclear
interactions relevant for modeling excitation energy transfer processes
in light harvesting systems is presented. The approach is based on
molecular dynamics (MD) calculations of classical correlation functions
of long-range contributions to excitation energy fluctuations and
a separate harmonic analysis and single-point gradient quantum calculations
for electron–intrachromophore vibrational couplings. A simple
model is also presented that enables detailed analysis of the shortcomings
of standard MD-based excitation energy fluctuation correlation function
approaches. The method introduced here avoids these problems, and
its reliability is demonstrated in accurate predictions for bacteriochlorophyll
molecules in the Fenna–Matthews–Olson pigment–protein
complex, where excellent agreement with experimental spectral densities
is found. This efficient approach can provide instantaneous spectral
densities for treating the influence of fluctuations in environmental
dissipation on fast electronic relaxation.