Thermodynamics of non-rigid molecules at high temperature

At high temperatures, the average amplitude of vibration of molecules becomes large. Anharmonicity and rotation-vibration interactions contribute a greater proportion of the thermodynamic functions as the amplitudes go up. In order to determine the effect of the large amplitude motions, the free energy is written as G = G classical + G correction, where G classical includes anharmonic and rotation-vibration terms and G correction is the approximate correction due to quantization. The formal expressions for G are written out for linear triatomic molecules and applied to C3 and CO2 as examples