High-quality reference values of the exciton coupling parameters for testing the ATC method were obtained using the electronic energy transfer (EET) method (see Figure ). The EET method is in principle more accurate than the ATC method because it can describe excitonic coupling between molecules at close ranges, where orbital overlap and other quantum effects are important. In the EET method, couplings between excited states of the isolated molecules are calculated via an unconverged Hamiltonian matrix for the dimer (constructed from the densities of the isolated molecules). An advantage of this perturbative approach is that intermolecular charge-transfer states cannot interfere with the evaluation of the excitonic coupling, which is not the case for diabatisation methods.