(367f) Transitioning from Empirical to Ab Initio Potentials for the Prediction of Thermodynamic Properties and Phase Equilibria

The application of theory for the prediction of thermodynamic properties and phase equilibria [1] has relied on approximate theoretical models, such as equations of state [2]. Some very sophisticated equations of state are available, which incorporate a high level of realistic molecular detail [3]. Nonetheless, molecular simulation [4] is arguably the preferred method because it minimises the number of theoretical assumptions. The choice of intermolecular potential is the primary theoretical assumption in conventional molecular simulation studies.

In common with equation of state calculations, the choice of intermolecular potential for use in molecular simulations is often based on empirical considerations [4]. For example, it is common to find the empirical Lennard-Jones potential used in elaborate force fields [4] that accurately determine the properties of macromolecules and proteins. Advances in computational chemistry mean that accurate potentials are being developed from first principles. [6] However, these ‘state-of-the-art’ ab initio potentials are largely confined to two-body interactions, whereas both thermodynamic properties and phase equilibria are noticeably affected by multi-body interactions. This means that purely empirical potentials, such as the (n-m) Lennard-Jones/Mie potentials can often be used to predict thermodynamic properties much more accurately than sophisticated ab initio potentials. In this work, we examine how ab initio potentials can be transitioned to accurately predict the properties of real fluids such as noble gases and water [6]. This involves incorporating multi-body effects such as three-body interactions and polarization to ab initio potentials. We demonstrate that ab initio potentials can be systematically used as the theoretical backbone for improvements in the quality of prediction of both thermodynamic properties and phase equilibria.

[1] R. J. Sadus, High Pressure Phase Equilibria of Multicomponent Fluid Mixtures, Elsevier, Amsterdam, 1992.

[2] Y. S. Wei and R.J. Sadus, AIChE J. 2000, 46, 169-196.

[3] P. Morgado, O. Lobanova, E.A, Mueller, G. Jackson, M. Almeida, and E. J. M Filipe, Mol. Phys. 2016, 114, 2597-2614.

[4] R. J. Sadus, Molecular Simulation of Fluids: Theory, Algorithms and Object-Orientation, Elsevier, Amsterdam, 1999.

[5] R. Hellmann E. Bich, and E. Vogel, Mol. Phys. 2008, 106, 813-825.

[6] M. Vlasiuk, F. Frascoli and R. J. Sadus, J. Chem. Phys. 2016, 145, 104501.