In the following, the error introduced by not optimizing the molecular orbitals is discussed. Since the CI energy of the first SCF iteration is the GASCI energy on the initial, unoptimized orbitals, it is possible to compare the GASCI total energies with the GASSCF energies. The energy difference between GASCI and GASSCF (E GASCI -E GASSCF ) for different number of interspace excitations, n exc , and different distances between the benzene fragments is shown in Figure .