The quantum mechanics-based polarizable force field for water simulations

We report here a new force field for water based solely on quantum mechanics (QM) calculations with no empirical data. The QM was at a high level, coupled cluster single double triple, for all orientations and distances for water dimer plus X3LYP density functional theory (DFT) on 19 larger water clusters. In addition, we included charge and polarization based on the polarizable charge equilibration method and nonbond interactions from DFT-D3 calculations on the H₂ and O₂ crystal. This model, denoted as RexPoN, provides quite excellent agreement with experimental (expr) data for the solid and liquid phase of water: T_melt = 273.3 K (expr = 273.15 K) and properties at 298 K: ΔH_vap = 10.36 kcal/mol (expr = 10.52), density = 0.9965 gr/cm³ (expr = 0.9965), entropy = 68.4 (J/mol)/K (expr = 69.9), dielectric constant = 76.1 (expr = 78.4), and ln D_s (self-diffusion coef) = -10.08 (expr = -11.24). Such an accurate force field for water will, we believe, be useful for full solvent calculations of electrocatalysis, where we can restrict QM water to just the first one or two layers involving reactions, using RexPoN to provide the polarization for a more distant solvent. Also, RexPoN may provide a better description of the solvent for proteins, DNA, polymers, and inorganic systems for applications to biomolecular, pharma, electrocatalysis (fuel cells and water splitting), and batteries where interaction with explicit water molecules plays a significant role.