  This article describes a mean-field theoretical model for Spin-Crossover
(SCO) materials and explores its implications. It is based on a simple
Hamiltonian that yields the high spin molar fraction as a function of
temperature and pressure, as well as a temperature-pressure phase diagram for
the SCO transition. In order to test the model, we apply it to the giant
Barocaloric Effect (BCE) of the SCO material [FeL$_2$][BF$_4$]$_2$ and
comprehensively analyse its behavior. We found that optical phonons are
responsible for 92\% of the total barocaloric entropy change. DFT calculations
show that these optical phonons are mainly assigned to the low frequencies
modes of vibration ($<400$ cm$^{-1}$), being associated to the Fe coordination.
