Acoustic crystal thermodynamic integration method

The acoustic crystal thermodynamic integration method is a generalization of the Einstein crystal method developed by Frenkel and Ladd. The name is derived from the acoustic branches of the phonon spectrum of the reference system. The method is designed to calculate the Helmholtz free energy of classical solid phases using Monte Carlo or molecular dynamics simulations. It has several advantages over the Einstein crystal method. For large systems, the Einstein crystal method suffers from very long correlation times near the zero coupling limit because the reference system breaks the overall translational symmetry of model systems. The acoustic crystal method does not break translational symmetry, so correlation times for the acoustic crystal are small. This makes the acoustic crystal method superior to the Einstein crystal method for large system sizes. Also the acoustic crystal method does not artificially introduce long-range order in low-dimensional systems.