Characterization of the static disorder in the B850 band of LH2

A systematic simulation study, guided by the expressions of the first-order perturbation theory, was performed for a model of the B850 band in the lig ht harvesting complex 2 of purple bacteria. General types of Gaussian disor der, which include diagonal, off-diagonal, coexisting but independent diago nal and off-diagonal, and correlated diagonal and off-diagonal disorder, we re considered. The effects of these types of disorder were studied by calcu lating four quantities that characterize the energy distribution of the thr ee lowest levels, varying the magnitude of the disorder. For all the cases considered, except for one of the correlated disorder models, a common scal ing relation was found among the four quantities, which is in reasonable ag reement with available low temperature experimental data. On the basis of t his fact and further physical reasoning, it is concluded that the disorder in the B850 is likely Gaussian and that comparable magnitudes of both diago nal and off-diagonal disorder coexist. Three distinct geometrical mechanism s were considered. and it was found that radial fluctuations in the transit ion dipole positions and fluctuations in the transition dipole angles are p lausible sources of the off-diagonal Gaussian disorder.