Modeling of mixing acetone and water: how can their full miscibility be reproduced in computer simulations?

The free energy of mixing of acetone and water is calculated at 298 K by means of thermodynamic integration considering combinations of three acetone and six water potentials. The Anisotropic United Atom 4 (AUA4) and Transferable Potential for phase Equilibria (TraPPE) models of acetone are found not to be miscible with any of the six water models considered, although the free energy cost of the mixing of any of these model pairs is very small, being below the mean kinetic energy of the molecules along one degree of freedom of 0.5RT. On the other hand, the combination of the Pereyra, Asar, and Carignano (PAC) acetone and TIP5P-E water models turns out to be indeed fully miscible, and it is able to reproduce the change of the energy, entropy, and Helmholtz free energy of mixing of the two neat components very accurately (i.e., within 0.8 kJ/mol, 2.5 J/(mol K), and 0.3 kJ/mol, respectively) in the entire composition range. The obtained results also suggest that the PAC model of acetone is likely to be fully miscible with other water models, at least with SPC and TIP4P, as well.