  The extent of exciton coherence in protein-pigment complexes has significant
implications for the initial light harvesting step in photosynthetic organisms.
In this work we model the main antenna protein of photosystem II, namely light
harvesting complex II (LHC-II), with a single-exciton Hamiltonian with sites
coupled via dipole-dipole interaction, with linear coupling to a dissipative
phonon bath. With appropriate parameters, Monte Carlo path integral (MCPI)
results of the exciton coherence length from 1 K to 500 K show that at
thermodynamic equilibrium, an exciton in LHC-II is localized mostly on 2 single
chlorophyll pigment sites, with persistent short-range coherence over the A2-B2
pair, A3-B3 pair and B1-B5-B6 triplet. Quasi-adiabatic path integral (QUAPI)
calculations of the subsystems mentioned above show a smooth, incoherent
relaxation towards thermodynamic equilibrium. The results obtained imply that
with the exception of small coherent subsystems at cryogenic temperatures,
excitons in LHC-II are more localized than in the analogous light harvesting
complex II (LH-II) of the purple bacterium Rs. molischianum, which may be
expected from the lower symmetry of the former.
