  Using the reduced density matrix formalism the exciton scattering in
light-harvesting systems of purple bacteria is calculated. The static disorder
(fluctuations of the site energies) as well as the dynamic disorder
(dissipation) is taken into account in this work. Circular aggregates with 18
pigments are studied to model the B850 ring of bacteriochlorophylls with LH2
complexes. It can be shown that the influence of dissipation may not be
neglected in the simulation of the time-dependent anisotropy of fluorescence.
Also an elliptical deformation of the ring could be essential.
