The PPP model is the most straightforward model for studying correlated electrons in πconjugated molecules. Like the Hückel model, it only considers the 2p z atomic orbitals (AOs) that are perpendicular to the molecular plane at each atomic site. However, unlike the Hückel model, the PPP model incorporates electron-electron (e-e) interactions using the zero differential overlap (ZDO) approximation, which ignores overlap integrals between 2p z orbitals on different atoms. The PPP model Hamiltonian in the real-space (RS) atomic orbitals (AO) picture reads: