  The continuous cooling of a white dwarf is punctuated by events that affect
its cooling rate. Probably the most significant of those is the crystallization
of its core, a phase transition that occurs once the C/O interior has cooled
down below a critical temperature. This transition releases latent heat as well
as gravitational energy due to the redistribution of the C and O ions during
solidification, thereby slowing down the evolution of the white dwarf. The
unambiguous observational signature of core crystallization - a pile-up of
objects in the cooling sequence - was recently reported. However, existing
evolution models struggle to quantitatively reproduce this signature, casting
doubt on their accuracy when used to measure the ages of stellar populations.
The timing and amount of the energy released during crystallization depend on
the exact form of the C/O phase diagram. Using the advanced Gibbs-Duhem
integration method and state-of-the-art Monte Carlo simulations of the solid
and liquid phases, we have obtained a very accurate version of this phase
diagram, allowing a precise modeling of the phase transition. Despite this
improvement, the magnitude of the crystallization pile-up remains
underestimated by current evolution models. We conclude that latent heat
release and O sedimentation alone are not sufficient to explain the
observations and that other unaccounted physical mechanisms, possibly $^{22}$Ne
phase separation, play an important role.
