Rovibrational energies of the hydrogen molecule in the electronic ground state have been derived from a complete vacuum ultraviolet (VUV) spectrum. This purely experimentally based approach has been chosen to compare experimentally based quantum-mechanical predictions for caloric properties of hydrogen in the ideal-gas state with approaches fitting smoothing functions to IR data and with ab-initio calculations of rovibrational energy levels. Results of this comparison were found to agree essentially within 0.01% in the isobaric heat capacity and give an indication for the uncertainty ideal gas properties. The given energy levels allow for the calculation of properties of ortho, para and normal hydrogen as well as for calculations regarding the equilibrium ortho / para distribution as a function of temperature. The results form the basis for the development of a numerically less demanding empirical representation of the ideal gas properties as basis for the development of a new reference equation of state.
