  Collective protein modes are expected to be important for facilitating energy
transfer in the Fenna-Matthews-Olson (FMO) complex, however to date little work
has focussed on the microscopic details of these vibrations. The nonlinear
network model (NNM) provides a computationally inexpensive approach to studying
vibrational modes at the microscopic level, whilst incorporating anharmonicity
in the inter-residue interactions which can influence protein dynamics. We
apply the NNM to the FMO complex and find evidence for the existence of
nonlinear discrete breather modes. These modes tend to transfer energy to the
highly connected core pigments, potentially opening up alternative excitation
energy transfer routes. Incorporating localised modes based on these discrete
breathers in the optical spectra calculations for FMO using ab initio site
energies and excitonic couplings can substantially improve their agreement with
experimental results.
