While CCSD(T) and higher-level coupled-cluster methods available today do a good job in describing the equilibrium structure and vibrational potential, an elaborate multireference configuration interaction calculations can describe the entire ground-state surface out to the dissociation limit , facilitating calculations of spectroscopy and chemical reactions of ozone. As such, the quantum chemical understanding and fidelity for the ground electronic state of O 3 is now at a mature level. Qualitatively, the challenge posed to electronic structure theory by ozone ultimately arises from its closely spaced highest-occupied (a 2 ) and lowest-unoccupied (b 1 ) molecular orbitals (HOMO and LUMO, respectively; see Fig. ), giving rise to the following wavefunction: