Combining rules

Abstract: The charges and Lennard-Jones parameters of the TIP3P water potential have been modified to improve its performance under the common condition for molecular dynamics simulations of using Ewald summation in lieu of relatively short nonbonded truncation schemes. These parameters were optimized under the condition that the hydrogen atoms do not have Lennard-Jones parameters, thus making the model independent of the combining rules used for the calculation of nonbonded, heteroatomic interaction energies, and limiting the number of Lennard-Jones calculations required. Under these conditions, this model provides accurate density (rho = 0.997 g/ml) and heat of vaporization (DeltaH(vap) = 10.53 kcal/mol) at 25 degrees C and 1 atm, but also provides improved structure in the second peak of the O-O radial distribution function and improved values for the dielectric constant (epsilon(0) = 89) and the diffusion coefficient (D = 4.0 x 10(-5) cm(2)/s) relative to the original parametrization. Like the original parameterization, however, this model does not show a temperature density maximum. Several similar models are considered with the additional constraint of trying to match the performance of the optimized potentials for liquid simulation atom force field to that obtained when using the simulation conditions under which it was originally designed, but no model was entirely satisfactory in reproducing the relative difference in free energies of hydration between the model compounds, phenol and benzene. Finally, a model that incorporates a long-range correction for truncated Lennard-Jones interactions is presented, which provides a very accurate dielectric constant (epsilon(0) = 76), however, the improvement in this estimate is on the same order as the uncertainty in the calculation.