where i, j, k and l run on the molecular orbitals, while µ and ν run on the atomic sites. The above Hamiltonian is written on the basis defined by the ground state configuration and the single, double, triple, etc. excited configurations obtained by promoting one, two, three, etc. electrons from the occupied to the virtual MOs. The resulting matrix is attacked by using efficient algorithms designed for storing and diagonalizing large matrices. The PPP Hamiltonian in Eq. 3 is much less sparse than on the RS basis. Nonetheless, as showed in Ref.