Computational Estimation of Temperature-Dependent Gas Entropy of Hydrogen Isotopologues

A validated computational framework is presented for estimating the temperature-dependent entropy of hydrogen isotopologues using first-principles calculations. By applying statistical thermodynamic models to vibrational and rotational parameters derived from density functional theory (DFT), results align closely with reference data from JANAF tables up to 2000 K, with a systematic difference at higher temperatures which arise from the limitations of this approach compared to the empirical fitting present in the JANAF reference data. Although high-accuracy rovibrational thermochemistry exists for the nonradioactive isotopologues, the contribution here is a reproducible plane-wave DFT to thermochemistry workflow designed to generate internally consistent entropy functions across isotopologue families, including tritium containing species where experimental handling is restricted. These findings support extending this approach to radioactive isotopologues (T₂, HT, DT), providing a safe and practical means of estimating temperature-dependent entropy reference data critical to fusion fuel cycle design.