where A E M is the free energy of the ideal Einstein molecule; ∆A id→I E M is the free energy difference between the ideal Einstein molecule and the interacting Einstein molecule (i.e, turning on the force field); and ∆A I E M →solid is the free energy difference between the interacting Einstein molecule and the solid (i.e, turning off the harmonic restraints). The advantage of EMM over ECM is the ab-sence of the need to calculate a free energy term associated with releasing the fixed reference point .