The improved Hulburt-Hirschfelder potential energy function is used as a proposal to emulate the behavior of intramolecular motion of a gas composed of diatomic molecules. In this work, the rovibrational energies of ClO molecule in the ground electronic state are obtained by solving the one-dimensional Schrödinger equation with the improved Hulburt-Hirschfelder potential energy curve. And then, the molar heat capacities, molar entropies, molar enthalpies and reduced Gibbs free energies of ClO macroscopic gas are calculated by the quantum statistical ensemble theory. The results are compared with those calculated by using RKR potential energy curve from experiment and Morse potential energy function. It is found that the macroscopic thermodynamic properties of ClO calculated by the improved Hulburt-Hirschfelder potential energy function are closer to the experimental values, which provides a new way to calculate the macroscopic thermodynamic quantities of diatomic gas based on molecular microscopic information.

Sulfur oxide (SO) is a kind of well-known diatomic molecule which becomes one of the major pollutants in the atmosphere. Mastering the heat capacities of SO is of great significance for elucidating its macroscopic evolution process. In the research of macroscopic systems, which are composed by many particles as well as several matters, it is an important approach to obtain macroscopic thermodynamic quantities of the system by constructing partition function from the microscopic information of molecule. For diatomic molecules in certain electronic state, the partition function can directly be obtained by calculating the rovibrational energies of the system to acquire the macroscopic molar heat capacities.<br>In this work, the contribution of rotational behavior to molar heat capacities is further considered. The first and foremost, on behalf of getting accurate microscopic information about SO, the potential energy curve including the long range region for the ground electronic state is constructed by using the simultaneous variational algebraic method (VAM) and Rydberg-Klein-Rees (RKR) method. And then, the full set of rovibrational energies of the system are acquired analytically. Ultimately, the molar heat capacities of SO molecule in the temperature range of 300-6000K are calculated through the partition function constructed by rovibrational energies. The above calculation depends only on the experimental vibrational energies, experimental rotational spectral constants and the dissociation energy of SO. Fortunately, through comparison of theoretical calculation results and experimental data, we find that the molar heat capacities of gaseous SO can be well predicted by employing the full set of rovibrational energies to describe the internal vibration and rotation of SO molecule. The idea of calculating the molar heat capacities by using the full set of rovibrational energies makes up for the shortcomings of ones through the approximate model to characterize the molecular rotational behavior in the previous work, and also provides opportunities for predicting the macroscopic thermodynamic quantities with limited experimental spectroscopy data.