  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.
