As a small example, diatomic oxygen in the triplet ground state ( 3 O 2 ) is differentiated by composition from the oxygen atom (O 1 ), by charge from a peroxide anion (O 2 -2 ), and by spin from the singlet excited state ( 1 O 2 ). Notably, additional information may be needed to distinguish between ground and excited states. To specify beyond this starting point, there are two natural definitions: one based on PES, and another based on the idea of chemical bonding (Figure ).