  Due to its high energy efficiency and environmental friendliness, solid-state
cooling based on the barocaloric (BC) effect represents a promising alternative
to traditional refrigeration technologies relying on greenhouse gases. Plastic
crystals displaying orientational order-disorder solid-solid phase transitions
have emerged among the most gifted materials on which to realize the full
potential of BC solid-state cooling. However, a comprehensive understanding of
the atomistic mechanisms on which order-disorder BC effects are sustained is
still missing, and rigorous and systematic methods for quantitatively
evaluating and anticipating them have not been yet established. Here, we
present a computational approach for the assessment and prediction of BC
effects in orientationally disordered materials that relies on atomistic
molecular dynamics simulations and emulates quasi-direct calorimetric BC
measurements. Remarkably, the proposed computational approach allows for a
precise determination of the partial contributions to the total entropy
stemming from the vibrational and molecular orientational degrees of freedom.
Our BC simulation method is applied on the technologically relevant material
CH$_{3}$NH$_{3}$PbI$_{3}$ (MAPI), finding giant BC isothermal entropy changes
($|\Delta S_{\rm BC}| \sim 10$ J K$^{-1}$ kg$^{-1}$) under moderate pressure
shifts of $\sim 0.1$ GPa. Intriguingly, our computational analysis of MAPI
reveals that changes in the vibrational degrees of freedom of the molecular
cations, not their reorientational motion, have a major influence on the
entropy change that accompanies the order-disorder solid-solid phase
transition.
