The RS diagonalization leads to exact full-CI results. However, in order to address larger systems and for a direct comparison with current quantum chemical approaches to triangulene-like systems, we adopt a molecular orbital (MO) approach to the PPP Hamiltonian. Specifically, in the Hartree Fock (HF) approximation the PPP Hamiltonian in Eq. 1 reduces to the one-electron Fock operator: