One important caveat in force field development is that the fundamental approximations in the functional form could make it impossible or inappropriate to reproduce certain physical properties. For example, it is well-known that classical models cannot reproduce the heat capacity due to the importance of nuclear quantum effects in high-frequency intramolecular and intermolecular degrees of freedom. Another relevant example is that all known simple three-point water models, i.e. those that use fixed partial charges, fail to reproduce the density anomaly at 4 • C even when they are fit to data for the temperature dependence in the density. In modeling the heat of vaporization, it is often necessary to apply post-hoc corrections to account for condensed phase polarization as well as nuclear quantum effects.