The minimal model of light harvesting complex with dipole-quadrupole interaction derived from ab initio calculation

A minimal, one-parameter model of the excitonic Hamiltonian for the light harvesting complex of purple bacteria (LH2) based on ab initio calculation of the excitonic energies is proposed. The set of input parameters contains positions of atoms in 27 bacteriochlorophyll pigments only. The excitation energies, transition dipole and quadrupole moments of the bacteriochlorophyll units were calculated by advanced multiconfigurational multireference method. In the work we investigate influence of the dipole-quadrupole interaction on the particularities of the absorption spectra and circular dichroism and on the wave-functions localization. We demonstrate that although the spectra changes are small the dipole-quadrupole interaction term leads, however, to almost complete separation of the system onto two subsystems: the small B800 ring of pigments and the large B850 ring. The latter observation is seemed essential for formulation of energy transport models in light harvesting complexes.