Traditionally, the prediction of both fluid phase equilibria and the thermodynamic properties of fluids [1] have relied on approximate theoretical models such as equations of state [2]. Equations of state have become increasingly sophisticated and many incorporate considerable molecular detail [3]. This has resulted in a considerable improvement in the accuracy of prediction, particularly for complex systems such as macromolecules. Nonetheless, molecular simulation [4] is arguably the method of choice for predictions because it can be applied with relatively few theoretical assumptions. Apart from ‘on the fly’ methods, such as Car-Parrinelo [5] molecular dynamics, the main assumption is the choice of intermolecular potential. Most commonly, the choice of intermolecular potential is based on empirical considerations, but increasingly accurate potentials are being developed [6] from first principles. In this work, we examine the accuracy of *ab initio* potentials to calculate both vapour-liquid equilibria and thermodynamic properties such as heat capacities, thermal expansion coefficient, speed of sound etc [7]. The analysis ranges from simple noble gases to promising results for molecular systems, such as water. The approach yields valuable insights into the key underlying interactions responsible for macroscopic properties. We demonstrate that using an *ab initio* approach provides a sound theoretical backbone for the systematic improvement of predictions via the addition of contributions such as three-body interactions and polarization.

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. 46, 169-196 (2000).

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

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

5. R. Car and M. Parrinello, Phys. Rev. Lett. 55, 2471-2474 (1985).

6. R. Hellmann E. Bich, and E. Vogel, Mol. Phys. 106, 813-825 (2008).

7. M. Vlasiuk, F. Frascoli and R. J. Sadus, J. Chem. Phys. 145, 104501 (2016).

Advances in molecular simulation , Challenges and advances in fluid phase equilibria