Unlike the Andersen barostat, the Parrinello-Rahman (PR) barostat allows the volume of the simulation cell to change anisotropically, which is well-suited for studying pressure-induced phase transitions of crystals. In the PR method, the simulation box is composed of three vectors a, b and c, which are allowed to have different lengths and orientations. Similar to Andersen's approach, the scaling of atomic coordinates can be expressed as