Description
There are many different models out there. Computational chemistry packages usually deal with the ideal-gas limit only, but there are models for hindered rotors and models for the harmonic limit and crystals.
I mostly want to predict the experimental enthalpies and entropies of molecules in the gas phase (this issue) and solution (see #26). As such, quasi-harmonic harmonic approximations are useful. Excellent models for supramolecular chemistry (mean absolute deviation of 2.1 kcal/mol) are given by Grimme (2012), together with a quasi-harmonic approximation to entropies. Li (2015) introduced a robust and theoretically sound quasi-harmonic estimate to enthalpies, with 1.8 kcal/mol of error for a particularly well-modeled system. Ribeiro (2011) offers some interesting numerical examples to test.