Evaluation and Refinement of the General AMBER Force Field for Nineteen Pure Organic Electrolyte Solvents

The performance of the general AMBER force field (GAFF) was evaluated by computing the density (rho), dielectric constant (epsilon), viscosity (eta), and enthalpy of vaporization (Delta H-vap) for 19 different organic solvents at 298 K and 1 atm. The force field performed very well for liquid densities, with deviations from experiment averaging around 3%. The performance of the force field was much worse for the other properties, with average absolute deviations of 10% for Delta H-vap, 35% for epsilon, and 132% for eta. A set of modified GAFF parameters were developed for each fluid using an optimization procedure to uniformly scale the Lennard-Jones parameters and partial charges. The modified parameters generally yielded more accurate properties than the original GAFF model, though discrepancies still remained for some liquids. Since these properties depend on the parameters of this class of force field in a complex and nonadditive manner, it is unlikely that any combination of parameters will yield a very high accuracy for all four properties. In particular, obtaining accurate dielectric constants and viscosities without compromising the liquid density or enthalpy of vaporization may require the use of a polarizable force field.