Orbital dependent complications for close vs well-separated electrons in diradicals.

This article focuses on the application of the Fermi–Löwdin-orbital self-interaction correction (FLOSIC) method to investigate the electronic structure of open-shell diradical systems, specifically ozone (O₃) and a polycyclic aromatic diradicaloid ((C₂₁H₁₃)₂). It demonstrates that standard density functional approximations (DFAs) such as local density approximation (LDA), generalized gradient approximation (GGA), and meta-GGA fail to predict the open-shell singlet ground state of ozone due to self-interaction errors, instead favoring a closed-shell singlet. In contrast, FLOSIC calculations reveal ozone’s ground state as an open-shell singlet with antiferromagnetic coupling between terminal oxygen atoms, consistent with multi-configuration wavefunction methods and experimental diamagnetism, quantifying approximately 26% biradical character. For the diradicaloid with well-separated unpaired electrons, both standard DFAs and FLOSIC correctly predict an open-shell singlet ground state, with FLOSIC providing enhanced spin localization. The study further discusses how FLOSIC-derived states and overlap integrals can inform model Hamiltonians for molecular magnets and highlights the method’s potential for addressing strongly correlated open-shell systems where conventional DFAs are inadequate.