The CD spectrum of each selected configuration was calculated employing an excitonic model in the matrix method formulation. This approach is based on the construction of an exciton Hamiltonian, whose parameters are derived by multiscale QM/MM calculations (see below for the details). Ab initio exciton models have proven successful in describing the optical properties, in particular the CD spectra, of several classes of multi-chromophoric systems. In such models, the exciton Hamiltonian Ĥex of N interacting chromophores is constructed on the n excitation energies E a i of each non-interacting chromophore (site energies), and on the electronic coupling V ab ij between two transitions of different chromophores as