To investigate the dimer-centred relaxation mechanisms in the crystal phase, we optimised the S 0 and S 1 geometries of the dimers with the shortest centroid distances (π -π stacked dimers) applying the QM/QM' approach as implemented in the fromage code. The QM (dimer) is represented by the algebraic diagrammatic construction ADC(2) /aug-cc-pVDZ method and QM' (environment) region was simulated at the second order (SCC-)DFTB method, employing the mio-1-1 set of Slater-Koster parameters parametrised for the tight-binding SCC-DFTB Hamiltonian. For the point charges in the environment, we used the ESP charges obtained at ωB97XD/6-31G(d) and PBE/6-31G(d) levels of theory, respectively. The DFTB calculations were performed with the DFTB+ program, and the ADC(2) computations with the Turbomole program. The lowest lying singlet and triplet states of the optimised dimers were computed at the ADC(2)/aug-cc-pVDZ level of theory.