The Three–Dimensional “Mercedes Benz” Model of Water†

Alan Bizjak¹, Tomaž Urbič¹, Vojko Vlachy^1,* and Ken A. Dill^2
1Faculty of Chemistry and Chemical Technology, University of Ljubljana, 1000 Ljubljana, Slovenia,
²Department of Pharmaceutical Chemistry, University of California, San Francisco, California, 94143-1204 USA.
 E-mail: vojko.vlachy@fkkt.uni-lj.si

Abstract
Previously, we and others have explored the “Mercedes Benz” model of water, a simplified 2-dimensional model that captures many of the qualitative features of water. Here, we introduce the 3-dimensional analogue of this model. Water molecules interact through Lennard-Jones attractions and repulsions, and through Gaussian orientation-dependent hydrogen bonding terms. In 3D MB model of water, hydrogen bonding structure is tetrahedral, hence more realistic than in the 2D version. We explore the model using NPT Monte Carlo simulations. Thermodynamic properties such as molar volume, heat capacity, isothermal compressibility, and the thermal expansion coefficient were studied as a function of temperature at fixed dimensionless pressure, P* = 0.12. These results indicate that the model satisfactorily captures the main experimental properties of real water, including the volumetric and thermal anomalies.

Keywords: Simple model of water, Monte Carlo simulation, thermodynamic properties.