Stability of tetraoxocyclobutane revised: Perturbation theory and density functional scheme

The stability of the singlet and triplet states of tetraoxocyclobutane (C4O4, the tetramer of carbon monoxide) has been investigated at various levels of theory. Second-order perturbation favors the singlet state, whereas the triplet and open-shell singlet states are computed to be more stable using the hybrid B3LYP density functional method. High-level CCSD(T) calculations reveal the perturbation theory to be insufficient, whereas the B3LYP method provides both qualitatively and quantitatively reliable results.