electrons from the occupied to the virtual MOs. The PPP Hamiltonian is then written on the basis defined by the different configurations and diagonalized in the CI approach (see ESI, Sec. S1). The PPP Hamiltonian on the CI basis is far less sparse than on the RS basis, so that full-CI is only feasible for very small molecules (up to ∼ 12 π-electrons on 12 atoms). However, as it will be shown below, the results converge quickly with the number of excitations, making it possible to obtain reliable results on fairly large systems.