Abstract: The molar heat capacity of ideal gas can be quantitatively calculated using quantum statistical thermodynamics. However, due to difficulty on determining degrees of freedom in liquid state, there is currently no method for directly computing the molar heat capacity of liquids. We noticed that the phase transition from gas to liquid has a limited effect on intramolecular vibrations, but converting the free translational and rotational degrees of freedom of gas molecules into quasi-vibrational degrees of freedom with very low frequency. Therefore, the molar heat capacity of a liquid can be quantitatively correlated with the molar heat capacity of the corresponding gas, calculated on the basis of evolution of quantum degrees of freedom.

Key words: Liquid, Transformation of motion, Heat capacity, Quantum statistical thermodynamics