The problem of calculating optical absorption and circular dichroism spectra for large aggregates of chromophores interacting through electronic transition dipoles is studied. The emphasis throughout is on relating band shapes of such aggregate spectra to band shapes of the consituent chromophores for all interchromophore coupling strengths. We argue that because of its close connections to an exact formulation, an approximation developed earlier (DGS model) predicts spectral band shapes much more accurately than other commonly used methods of calculating aggregate spectra. This is particularly true for intermediate or strong coupling, i.e., for interchromophore coupling strengths comparable to or greater than the bandwidths of the individual chromophores. A number of calculations of optical absorption and circular dichroism spectra for simple helical polymers have been carried out using the DGS model. These calculations exploit the formal identity of the DGS model to the rsquo;rsquo;cellular disorderrsquo;rsquo; problem of solid state physics and the existence of well established methods for solving this problem. The theoretical spectra found in this way differ considerably from those found using other methods, especially in intermediate and strong coupling. Consequences of these results for several polymers of biological interest are discussed.
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