ADCHα-I population analysis and constrained dipole moment density functional theory in force fields for molecular simulations.

A new population analysis, ADCHα-I, based on the interpolation between the Hirshfeld (H) and the iterative Hirshfeld (H-I) methods through a parameter α and on the atomic dipole moment corrected Hirshfeld (ADCH) methodology is proposed, in combination with the constrained dipole moment density functional theory (CD-DFT) previously developed, to determine the charge distributions of force fields. Following this approach, the electronic density of the isolated molecule is determined for the value of the dipole moment that reproduces the experimental dielectric constant, in order to incorporate through this property the effects of the surrounding molecules in the liquid, and to carry on this information to the molecular simulation, the new population analysis is built to obtain the set of charges that reproduces this dipole moment. By selecting α = 1/2, one is led to charges that are larger than the ones obtained through H and ADCH and smaller than those of H-I and that incorporate, at the local level, information about the response of isolated atoms to donate or to accept charge, which is not considered in ADCH. The results obtained for several liquid properties indicate that the combination of CD-DFT with this population analysis leads to a good description of the charge distributions in force fields used in molecular simulations.

Parametrization with Explicit Water of Solvents Used in Lithium-Ion Batteries: Cyclic Carbonates and Linear Ethers.

Molecular dynamics simulations are performed to study carbonates and ethers that are widely used as electrolytes in energy storage devices. The first type contains in their molecular geometry a hydrocarbon tail of ethylene, propylene, and butylene whereas in the second type, the tail comprises 1,2-dimethoxyethane and 1,2-diethoxyethane. The evaluation of optimized potential for liquid simulations (OPLS), CHARMM, and GROMOS force fields for some of the solvents shows poor agreement with experimental thermodynamic and transport properties leading us to parameterize those solvents using the OPLS parameters as the starting point. A systematic procedure that uses the solubility of the solvents as the target property in simulations with explicit water is applied. The transferability of the parameters of the smallest cyclic or linear molecules was used to simulate systems with longer hydrocarbon chains. The optimized parameter reproduce the experimental solubility of butylene carbonate and 1,2-diethoxyethane in water. The interaction parameters were used to obtain the self-diffusion coefficients of ions of the salt LiPF6 at 1 M concentration in mixtures with ethylene carbonate or propylene carbonate. The simulation results for pure components and mixtures with the new parameters are in excellent agreement with the experimental data.

Constrained dipole moment density functional theory for charge distributions in force fields for the study of molecular fluids.

A new procedure, based on electronic structure calculations that only requires a dipole moment value for a given molecule as input and, from which the charges for all the atoms in it are uniquely determined, is developed and applied to the study of molecular fluids with classical dynamics. The dipole moment value considered for the isolated molecule is the one that reproduces the dielectric constant of its corresponding fluid. Following previous work, the Lennard-Jones parameters are determined to reproduce the liquid density and the surface tension at the liquid-vapor interface. The force field thus obtained leads to a reasonable description of several properties such as heats of vaporization, self-diffusion coefficients, shear viscosities, isothermal compressibilities, and volumetric expansion coefficients of pure substances.