  Thermal equations of state (EoS) are essential in several scientific domains.
However, experimental determination of EoS parameters may be limited at extreme
conditions, therefore, {\it ab~initio} calculations have become an important
method to obtain them. Density Functional Theory (DFT) and its extensions with
various degrees of approximations for the exchange and correlation (XC) energy
is the method of choice, but large errors in the EoS parameters are still
common. The alkali halides have been problematic from the onset of this field
and the quest for appropriate DFT functionals for such ionic and relatively
weakly bonded systems has remained an active topic of research. Here we use DFT
+ van der Waals functionals to calculate vibrational properties, thermal EoS,
thermodynamic properties, and the B1 to B2 phase boundary of NaCl. Our results
reveal i) a remarkable improvement over the performance of standard Local
Density Approximation and Generalized Gradient Approximation functionals for
all these properties and phase transition boundary, as well as ii) great
sensitivity of anharmonic effects on the choice of XC functional.
