This can be explained as follows: Due to hermiticity (compare equation 22) we have Stack of benzene molecules. The next example are two stacks of benzene at a distance of 3.0 Å with five and ten molecules, respectively. The stack of ten molecules is displayed in Figure . In both cases the orbitals were fragment-localized, i.e. natural, Hückel-like orbitals on each benzene. The active space contained all the π and π * orbitals, i.e. a CAS (30,30) for five benzene and a CAS(60,60) for ten benzene molecules. As we have shown in a previous publication, the system as a whole is dominated by excitations inside each fragment. Additionally incorporating single excitations between the fragments is enough to recover Full CI energy.