calculation of the

• properties of single molecules
• electronic excited states and the electronic spectrum of selected clusters as supermolecules

• behavoir of a single molecule is strongly altered if simultaneously the nearest stack neighbor is present:
  strong intermolecular interactions
• interaction between the molecules mainly determined by the intermolecular overlap of the -wavefunctions (see figure right)

• intermolecular overlap not only determined by the separation of the nearest stack neighbors, but also by the longitudinal and transversal offset (see figure right)
• such offsets are characteristical for a given material:
  derivatives, e.g. PTCDA and MePTCDI, share the same molecular body, but their particular substituents induce a different molecular alignment in the crystals

• electronic excited states can be represented in terms of molecular and charge-transfer excitations
• extraction of interactions between molecules, transition dipole moments of basis excitations
• comparison to cluster described with nearest neighbor exciton models