It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and through a thermodynamic cycle, it can usually be calculated. But what about our knowledge of entropy limits our ability to measure it? Without using straight empirical data, why is using force field and electronic interactions not very reliable? Many reactions face this problem, like the mixing of liquids.