A new extension of classical molecular dynamics: An electron transfer algorithm

Nonpolarizable Force Fields through the Self-Consistent Modeling Scheme with MD and DFT Methods: From Ionic Liquids to Self-Assembled Ionic Liquid Crystals

A key to achieve the accuracy of molecular dynamics (MD) simulation is the set of force fields used to express the atomistic interactions. In particular, the electrostatic interaction remains the main issue for the precise simulation of various ionic soft materials from ionic liquids to their supramolecular compounds. In this study, we test the nonpolarizable force fields of ionic liquids (ILs) and self-assembled ionic liquid crystals (ILCs) for which the intermolecular charge transfer and intramolecular polarization are significant. The self-consistent modeling scheme is adopted to refine the atomic charges of ionic species in a condensed state through the use of density functional theory (DFT) under the periodic boundary condition. The atomic charges of the generalized amber force field (GAFF) are effectively updated to express the electrostatic properties of ionic molecules obtained by the DFT calculation in condensed phase, which improves the prediction accuracy of ionic conductivity with the obtained force field (GAFF-DFT). The derived DFT charges then suggest that the substitution of a hydrophobic liquid-crystalline moiety into IL-based cations enhances the charge localization of ionic groups in the amphiphilic molecules, leading to the amplification of the electrostatic interactions among the hydrophilic/ionic groups in the presence of hydrophobic moieties. In addition, we focus on an ion-conductive pathway hidden in the self-assembled nanostructure. The MD results indicate that the ionic groups of cation and anion interact strongly for keeping the bicontinuous nanosegregation of ionic nanochannel. The partial fractions of hydrophilic/ionic and hydrophobic nanodomains are then quantified with the volume difference from referenced IL systems, while the calculated ionic conductivity decreases in the self-assembled ILCs more than the occupied volume of ionic nanodomains. These analyses suggest that the mobility of ions in the self-assembled ILCs remains quite restricted even with small tetrafluoroborate anions because of strong attractive interaction among ionic moieties.

Self-Consistent Scheme Combining MD and Order-N DFT Methods: An Improved Set of Nonpolarizable Force Fields for Ionic Liquids

The nonpolarizable force field of ionic liquids is tuned by using the self-consistent scheme of molecular dynamics (MD) simulation and first-principles calculation based on the order-N density functional theory (DFT). The atomic charges are determined by using the whole MD cell for DFT calculation and accounts effectively for the many-body effects of charge transfer and intramolecular polarization. The charges represent effective interactions in the condensed phase within the framework of the nonpolarizable force field and can be an alternative for an explicitly many-body model incorporating, for example, polarizability. Here we demonstrate the performance of nonpolarizable force field determined with the MD-DFT self-consistent scheme in imidazolium-, pyrrolidinium-, and ammonium-based ionic liquids. The variation ranges of molecular charges are much larger with the compositions of the ionic liquid than with the thermodynamic conditions, and the charge-ordering structures become systematically weaker with the effective charges. For energetic properties, while the calculated heat of vaporization depends on the atomic and molecular charges, the corresponding heat capacity is not strongly affected by the DFT-based variation. For transport properties, the self-diffusion coefficient, electrical conductivity, and viscosity vary much more in the self-consistent scheme. The effective DFT charge is observed to enhance the fluidity of ionic liquids and improve the accuracy of electrical conductivity and viscosity. This is due to the weakened interactions among the ions, and the too slow motions observed with a full-charge model are well corrected through the iteration of MD and DFT. We therefore conclude that the set of nonpolarizable force fields obtained with the MD-DFT self-consistent scheme leads to better description of transport properties of ionic liquids.