Cooling is at the heart of modern technology, from storing and transporting food and medicine to computing. Yet current refrigeration technologies rely heavily on environmentally harmful refrigerants. For this reason it is vital to investigate alternative solid-state cooling methods such as barocaloric refrigeration, which involves cycling a carefully designed material between high and low pressures in such a way that it pumps heat. In order to develop materials that are useful for such technologies, we need to understand at the atomic level the mechanism for their behaviour under pressure. In this proposal we aim to study the phonon density of states as a function of pressure and temperature in Ammonium Sulfate across the phase transition believed to drive a giant barocaloric effect.