The structural thermodynamic model is extended to cover the excited and ionized states of diatomic molecules using the atom-in-molecule approach, which is conventional in chemistry. The relative chemical potentials of H-2(+) and H-2(-) (chi(SI)(di) (H-2(+), g) = 1.0 and chi(SI)(di) (H-2(-), g) = 0) are recommended as standard values.