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The Frenkel exciton Hamiltonian is at the heart of many simulations ofexcitation energy transfer in molecular aggregates. It separates the aggregateinto Coulomb-coupled monomers. Here it is shown that the respective parameters,i.e. monomeric excitation energies and Coulomb couplings between transitiondensities, can be efficiently calculated using time-dependenttight-binding-based density functional theory (TD-DFTB). Specifically, Coulombcouplings are expressed in terms of self-consistently determined Mullikentransition charges. The determination of the sign of the coupling requires anadditional super-molecule calculation. The approach is applied to two dimersystems. First, formaldehyde oxime for which a detailed comparison withstandard DFT using the B3LYP and the PBE functionals is provided. Second, theCoulomb coupling is explored in dependence on the intermolecular coordinatesfor a perylene bisimide dimer. This provides structural evidence for thepreviously observed biphasic aggregation behavior of this dye.

A New Efficient Method for Calculation of Frenkel Exciton Parameters in Molecular Aggregates