In ECM, the free energy is calculated by: where A EC F C M is the free energy of the Einstein crystal (EC) with a fixed center of mass (FCM); ∆A EC→I EC is the free energy difference between the Einstein crystal (EC) and the interacting Einstein crystal (IEC), i.e., the free energy difference in a transformation where the force field is progressively turned on throughout the calculation path. ∆A I EC→SF C M is the free energy difference between the IEC and the solid with a fixed center of mass (SFCM), i.e, turning off the harmonic restraints; and ∆A r elease C M is the free energy of release of the center of mass (CM).