Free energy calculations are important for determining the relative stability of different competing phases, and hence map phase diagrams of model systems of interest. For example, they can be used to understand the phase behaviour materials at high pressure or low temperature. This is useful for a wide range of technologies, e.g. pharmaceuticals. This work concerns a new method for calculating the free energy of crystalline materials. The aim was to further develop a novel 'self-referential' technique so that it can be used quite efficiently and generally for crystals. The first, and key, outcome of this work[1] demonstrated how the self-referential technique could be made much more efficient by using a mathematical procedure called 'thermodynamic integration'. The second and third outcomes demonstrates that the self-referential technique can be applied quite generally to molecular crystals with internal degrees of freedom[2,3], i.e. essentially arbitrary bulk molecular crystals. For example, the method has recently been applied to calculate the free energy of gas hydrates used to store hydrogen[4].
However, it is now expected that the self-referential method is not restricted to molecular crystals, but can be used to calculate the free energy of any equilibrium system, including confined cyrystals, fluids, liquid crystals, etc, and work is ongoing to demonstrate this. If true, this would yield a very valuable tool for the whole molecular simulation community.
[1] The self-referential method combined with thermodynamic integration, Sweatman, M. B. , Atamas, A. A. & Leyssale, J-M. Feb-2008 In : Journal of Chemical Physics. 128, 6, 10.
[2] The self-referential method for linear rigid bodies: Application to hard and Lennard-Jones dumbbells, Sweatman, M. B. , Atamas, A. & Leyssale, J-M. Jan-2009 In : Journal of Chemical Physics. 130, 2, p. 024101.
[3] New techniques for simulating crystals, Sweatman, M. B. 2009 In : Molecular Simulation. 35, 10-11, p. 897.
[4] Monte Carlo calculations of the free energy of ice-like structures using the self-referential method, Atamas, A. , Koudriachova, M. V. , de Leeuw, S. W. & Sweatman, M. B. 2011 In : Molecular Simulation. 37, 4, p. 284.