Intermolecular excitonic interactions in one-dimensional molecular aggregates are strong enough to delocalize the electronic excitation coherently over a large number of molecules. Model calculations usually make a number of assumptions, for example that only the nearest neighbor molecules are involved or that the dipole-dipole coupling has a dependence of 1/R(3). Here, the electronic structure of the PIC excitations is investigated on the basis of quantum chemical calculations, allowing the validity of the assumptions to be assessed.