Complete exciton Hamiltonian for photosynthetic pigment-protein complexes

The standard exciton theory used for describing spectroscopy of molecular complexes usually assumes an infinite exciton lifetime, coming from complete isolation of Hamiltonian blocks describing a different number of excitations. We focus on extending the model by including environment-induced non-resonant couplings in the Hamiltonian, which couple different numbers of excitons. We analyze their impact on the absorption spectra of two small light-harvesting complexes: Fenna-Matthews-Olson and fucoxanthin-chlorophyll protein. Quantum mechanics calculations of the pigment properties, combined with an electrostatic description of the protein, allowed us to construct the full Hamiltonian and calculate the exciton lifetimes and the absorption spectra. The resulting off-resonant terms in the Hamiltonian are significant and cannot be excluded from calculations of linear (or non-linear) spectra.