Ideal Gas Helmholtz Energy Model Implementation Based on Quantum Chemical Data and Application to Equation of State Modeling

Ideal gas thermodynamic properties are essential for many industrial processes. In this work, an implementation of an ideal gas Helmholtz energy contribution model based on statistical mechanics and quantum chemistry ab initio data is given. The energy storage properties of individual molecules are described in the model by the classical rigid-rotor and harmonic oscillator─for the independent normal modes. Quantum mechanical calculations using density functional theory were performed for about 21,500 molecules to determine the vibrational and rotational energy levels. The model predictions were validated using experimental isobaric heat capacity data from the literature for approximately 1,000 molecules. The model yields average deviations of 0.5–8%, with over 91% of the molecules showing deviations below 5%. We demonstrate the applicability of the ideal gas model in combination with a molecular-based equation of state, the SAFT-VR Mie (J. Chem. Phys. 2013 139, 154504), to describe different thermodynamic properties.