ECM can be difficult to implement because of the need for a fixed center of mass, so our work here is based on an alternative approach which is easier to implement. When particles move in ECM, the lattice needs to be moved because the center of mass is fixed . Our method of choice, the Einstein Molecule Method (EMM, see Fig. )), fixes a single atom in the lattice instead of the center of mass and is more easily implemented than ECM because of the relative difficulty of introducing center of mass restraints into existing simulation packages . EMM has been used to predict phase diagrams of TIP4P and SPC/E water models , free energies of ice polymorphs, solid methanol and toy systems , and the solubilities of potassium and sodium chlorides .