On the parameterization of vibronic Hamiltonians for molecular aggregates using absorption line-shapes as an input.

Absorption line-shapes of molecular aggregates are often calculated using a simple form for a vibronic Hamiltonian. Parameters which enter into the model are usually taken from measured spectra. Here, we address the question in how far different sets of input parameters used to calculate the spectra lead to similar spectral features. Therefore, we first present analytical expressions which illustrate the relation between the various parameters such as Huang-Rhys factors, electronic coupling elements, and aggregate size. Numerical calculations show that identical peak ratios and spectra can be obtained for different sets of parameters. This illustrates the basic difficulties in extracting reliable molecular information from a comparison of calculated and measured spectra.