I. INTRODUCTION II. METHODS 2 We simulated a cube containing a pair of PIC (positively charged) molecules and 2150 TIP5 10 water molecules. We did not use periodic boundary conditions to avoid artefacts from the Coulombic interaction with the mirror images. Instead reflecting boundaries kept the molecules from escaping the box by reversing the normal velocity component whenever the For the classical MD simulation we used the model of rigid rotors which can be easily combined with quantum calculations to obtain electronic excitations and coupling matrix elements 9 . Since also the position of the ethyl groups is fixed we have to distinguish not only two stereoisomers but also a fully C2-symmetric form and another form where the ethyl-groups break this symmetry. A possible interconversion between these conformations was not taken into account. Since the discovery of the so called J-band 1,2 , an unusual sharp absorption band which is characteristic for the aggregation of the classical sensitizing dye 1,1’-diethyl-2,2’- cyaninchloride (pseudoisocyanine) a large amount of experimental and theoretical work ad- dressed the investigation of molecular aggregates and their red shifted J-bands. At lower concentration blue shifted H-bands were observed which were attributed to molecular dimers, the smallest possible aggregates. Molecular modelling of the J-aggregates is difficult due to the fact that the PIC molecule is a cation and therefore the Coulombic interactions as well as dielectric shielding have to be taken into account carefully. For the formation of larger aggregates the counterions are important whereas this seems not the case for the smaller H-aggregates 3 . Therefore we started the simulation of PIC aggregates by a detailed inves- tigation of the dimer. From the analysis of the experimental spectrum in water 4 it was deduced that both excitonic components contribute with an intensity ratio of 2:1. This can not be explained 5 by dimer models 4,6 where the dipole moments are almost parallel or antiparallel as it is the case for the common brickwork or ladder models which are found in the literature for the J-aggregate 7,8 . Another focus of our investigations concerns the contribution of local Coulombic interactions to the inhomogeneous broadening of the site energies and its importance in comparison to intramolecular vibrations.