Among quantum chemists, ozone has a notorious history, owing to its strong diradical character causing significant difficulties in calculations of its ostensibly simple ground-state molecular properties. An early 1989 study by the Bartlett group and collaborators found that the CCSD+T(CCSD) method predicted that the molecular equilibrium structure of ozone would have C s symmetry (that is, the asymmetric stretching harmonic frequency computed by this method is imaginary), a finding that led to a search for better treatment of non-iterative triple excitations, ultimately leading to the well-known CCSD(T) method .