Abstract

Simulations of water started around 1970 [1,2]. Intermolecular interactions were described using the Lennard-Jones (LJ) potential and partial charges as first suggested in 1933 [3]. Potential parameters were determined to reproduce the experimental density and the energy of liquid water. The study of phase equilibria arrived later: vapor-liquid equilibria in the 90s and fluid-solid equilibria in the XXI century and that allowed the proposal of improved water models as for instance TIP4P /2005 [4]. This model is not able to describe all properties of water and the reasons for this failure will be discussed. However, it allows to study a number of interesting physical problems as nucleation, the quasi-liquid layer and the possible existence of a liquid- liquid transition in supercooled water. Electrolytes in water are both important and interesting. In simulations they are commonly described using integer charges and single LJ centers for the ions but the results are far from being satisfactory. It will be shown, that the use of scaled charges for the ions along with TIP4P /2005 (as done in the Madrid force field [5] ) allows for a decent (although still not fully satisfactory) description of electrolyte solutions for properties such as densities, maximum in density, structure, freezing point depression and transport properties. Probably we reached the limit of can be done with simple non-polarizable force fields in modeling water and electrolyte solutions. To go beyond that we need a more sophisticated description ( including polarizability and/or ab initio input) . However, for certain type of problems the simple approach can be enough to learn some physics. [1] J. A. Barker and R. O. Watts,Chem.Phys.Lett.,3,144,(1969) [2] A. Rahman and F.H. Stillinger,J.Chem.Phys.,55,3336,(1971) [3] J. D. Bernal and R. H. Fowler,J.Chem.Phys.,1,515,(1933) [4] J.L.F.Abascal and C.Vega, J.Chem.Phys., 123 ,234505,(2005) [5] I.M.Zeron,J.L.F.Abascal,C.Vega,J.Chem.Phys.,151 134504 (2019)