Photosynthetic processes are driven by sunlight. Too little of it and the photosynthetic machinery cannot produce the reductive power to drive the anabolic pathways. Too much sunlight and the machinery can get damaged. In higher plants,
the major Light Harvesting Complex (LHCII) efficiently absorbs the light energy, but
can also dissipate it when in excess (quenching). In order to study the dynamics related to the quenching process but also the exciton dynamics in general, one needs to accurately determine the so-called spectral density which describes the coupling
between the relevant pigment modes and the environmental degrees of freedom. To
this end, Born–Oppenheimer molecular dynamics simulations in a quantum mechanics/molecular mechanics (QM/MM) fashion utilizing the density functional based tight
binding (DFTB) method have been performed for the ground state dynamics. Subsequently, the time-dependent extension of the long-range-corrected DFTB scheme
has been employed for the excited state calculations of the individual chlorophyll-a
molecules in the LHCII complex. The analysis of this data resulted in spectral densities showing an astonishing agreement with the experimental counterpart in this rather
large system. This consistency with an experimental observable also supports the accuracy, robustness, and reliability of the present multi-scale scheme. In addition, the
resulting spectral densities and site energies were used to determine the exciton transfer rate within a special pigment pair consisting of a chlorophyll-a and a carotenoid
molecule which is assumed to play a role in the balance between the light harvesting
and quenching modes.
The fragment molecular orbital method combined with density-functional tight-binding and periodic boundary conditions
