In this work, we present a new four-site potential for methanol, MeOH-4P, fitted to reproduce the dielectric constant ε, the surface tension γ , and the liquid density ρ of the pure liquid at T = 298.15 K and p = 1 bar. The partial charges on each site were taken from the OPLS/2016 model with the only difference of putting the negative charge on the fourth site ( M) instead of on the O atom, as done in four-site water models. The original Lennard-Jones (LJ) parameters of OPLS/2016 for the methyl moiety (Me) were modified for the fitting of ρ and γ, whereas the parameters of the TIP4P-FB water model were used for the O atom without change. Taking into account the energetic cost of the enhanced dipole relative to the isolated molecule, the results from simulations with this model showed good agreement with experiments for ρ, α , κ, C , and Δ H. Also, the temperature dependence of γ and ε is satisfactory in the interval between 260 and 360 K, and the critical point description is similar to that of OPLS/2016. It is shown that orientational correlations, described by the Kirkwood factor G, play a prominent role in the appropriate description of dielectric constants in existing models; unfortunately, the enhancement of the dipole moment produced a low diffusion coefficient D; thus, a compromise was required between a good reproduction of ε and an acceptable D. The use of a fourth site resulted in a significant improvement for water-methanol mixtures described with TIP4P-FB and MeOH-4P, respectively, but required the modification of the LJ geometric combination rule to allow a good description of the methanol molar-fraction dependence of ρ, ε, and methanol (water) diffusion coefficients D ( D) and excess volume of mixing Δ V in the entire range of composition. The resulting free energy of hydration Δ G shows excellent agreement with experiments in the interval between 280 and 360 K.

The Pb2+ presents unique hydration features that make the experimental characterization and its theoretical modeling challenging: classical molecular dynamics (MD) with standard force-fields fails to produce the experimentally determined diffusion coefficient and the EXAFS spectrum. Here we study the hydration of Pb2+ in aqueous solution employing a polarizable model compatible with the MCDHO water model. The MCDHO FF for the Pb2+–water interaction was fitted to reproduce the configurations and interaction energies of various [Pb­(H2O) n ]2+ clusters obtained with ab initio calculations, with n = 4, 6, and 8. Its use in classical MD simulations yielded qualitative agreement with Born–Oppenheimer molecular dynamics of gas-phase hydrated clusters and MD simulations of the aqueous solution resulted in good agreement with the experimental D Pb2+ and EXAFS spectrum. Analysis of the MD trajectories revealed a labile and very dynamic hemidirected first hydration shell in the aqueous solution with a non-well-defined coordination number CN; nonetheless, it was found that the more probable hydration structures have either 3 or 4 water molecules directly bound to the Pb2+ with another 3 or 2 at slightly larger distances. The simulations of the gas-phase [Pb­(H2O)29]2+ cluster were found to capture the main structural features of the diluted aqueous solution.

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The liquidus temperature curve that characterizes the boundary between the liquid methanol/water mixture and its coexistence with ice Ih is determined using the direct-coexistence method. Several methanol concentrations and pressures of 0.1 MPa, 50 MPa, and 100 MPa are considered. In this study, we used the TIP4P/Ice model for water and two different models for methanol: OPLS and OPLS/2016, using the geometric rule for the Lennard-Jones cross interactions. We compared our simulation results with available experimental data and found that this combination of models reproduces reasonably well the liquidus curve for methanol mole fractions up to xm=0.3 at p=0.1 MPa. The freezing point depression of these mixtures is calculated and compared to experimental results. We also analyzed the effect of pressure on the liquidus curve, and we found that both models also reproduce qualitatively well the experimental decreasing of the liquidus temperatures as the pressure increases.