On the complementarity of the harmonic oscillator model and the classical Wigner–Kirkwood corrected partition functions of diatomic molecules

The vibrational and rovibrational partition functions of diatomic molecules are considered in the regime of intermediate temperatures. The low temperatures are those at which the harmonic oscillator approximation is appropriate, and the high temperatures are those at which classical partition function (with Wigner–Kirkwood correction) is applicable. The complementarity of the harmonic oscillator and classical integration over the phase space approaches is investigated for the CO and H+2 molecules showing that those two approaches are complementary in the sense that they smoothly overlap.