Integral equation study of the effects of rotational degrees of freedom on properties of the Mercedes–Benz water model

Abstract

We developed an analytical theory for studying of rotational degrees of freedom of a simple model of liquid water. Wertheim's integral equation theory (IET) for associative liquids was applied to the Mercedes-Benz (MB) model, which is among the simplest models of water. The MB water molecules are modeled as 2-dimensional Lennard-Jones disks with three hydrogen bonding arms arranged symmetrically, resembling the MB logo. IET is based on the orientationally averaged version of the Ornstein-Zernike equation. In varying of rotational degrees of freedom we use different averaging then used before. By holding one of the temperatures constant and varying the other one, we investigate the effect of faster motion in the corresponding degrees of freedom on the properties of the simple water model and how well IET reproduce computer simulation results. The pair correlation function of the model water at high rotational or translation temperatures is correctly predicted. IET results are in good agreement with the Monte Carlo values of the pressure, energy, heat capacity. A major advantage of these theories is that they require orders of magnitude less computer time than the Monte Carlo simulations.