In the ECM, and in this work, the absolute free energy of the molecular solid is found by designing a path where force field terms are progressively turned on, and the harmonic potential position restraints are turned off. The fixed center of mass is important to avoid a quasidivergence issue when calculating the free energy term of releasing the system from the harmonic position restraints, but the contribution of the fixed center of mass needs to be included in the cycle to obtain the correct absolute free energy for the system (Fig. ) .