I am currently lecturing on statistical thermodynamics to our second year students, and I’ve got to the point where I am talking about the calculation of thermodynamic properties, like entropy and Gibbs free energy. Where possible I like to relate calculated properties to experimental results, and I seem to have come up against a problem here, so I thought I would mention it here in case someone with a better understanding of the subject than me reads it, and gives me some pointers. Heat capacities can be compared directly; here one is calculating dU/dT, and that works fine, and the agreement is impressive for rare gases and some diatomics, like nitrogen. Even entropies can be compared with tabulated values, but what about Gibbs free energies? From a chemical viewpoint, we talk about changes in Gibbs energy, not about absolute values. Further, Gibbs free energies of formation of elements in their standard states are defined as zero, so what are we comparing with when we calculate the Gibbs free energy of a nitrogen molecule using statistical thermodynamics? Or is there a subtle point I’m missing? Advice and pointers to reading would be gratefully received!