where the relative dielectric constant ε r is set to 2 to represent a typical organic medium. The RS PPP Hamiltonian is written on the basis that encompasses all possible states generated by distributing the n π-electrons among the N atoms. Specifically, the basis states are selected as eigenstates of the z-component spin operator Ŝz . Accordingly, doublet states can be singled out by diagonalizing the PPP Hamiltonian in both S z = 0.5 and S z = 1.5 subspaces and looking for states that appear exclusively in the S z = 0.5 subspace.