Ion transport across solid-state ion channels perturbed by directed strain.

We combine quantum-chemical calculations and molecular dynamics simulations to consider aqueous ion flow across non-axisymmetric nanopores in monolayer graphene and MoS2. When the pore-containing membrane is subject to uniaxial tensile strains applied in various directions, the corresponding permeability exhibits considerable directional dependence. This anisotropy is shown to arise from directed perturbations of the local electrostatics by the corresponding pore deformation, as enabled by the pore edge geometries and atomic compositions. By considering nanopores with ionic permeability that depends on the strain direction, we present model systems that may yield a detailed understanding of the structure-function relationship in solid-state and biological ion channels. Specifically, the observed anisotropic effects potentially enable the use of permeation measurements across strained membranes to obtain directional profiles of ion-pore energetics as contributed by groups of atoms or even individual atoms at the pore edge. The resulting insight may facilitate the development of subnanoscale pores with novel functionalities arising from locally asymmetric pore edge features.

A systematic approach for development of an OPLS-like force field and its application to hydrofluorocarbons.

A systematic, formal approach to optimization of force field parameters for molecular simulations is presented. The procedure is based on response surface mapping methodology that allows simultaneous parameter optimization against multiple property targets while constraining the number of required computationally expensive numerical experiments. The approach was implemented for prediction of vapor-liquid equilibrium properties of alkanes, alkenes, and their fluorinated derivatives via Monte Carlo molecular simulations. To further reduce computational costs, a bootstrap procedure that involves a sequence of parameter optimization for four pairs of compounds (ethane and propane, ethene and propene, perfluoroethane and perfluoropropane, and 2,3,3,3-tetrafluoropropene and (E)-1,3,3,3-tetrafluoropropene) was used. The results of simulations utilizing the optimized force field parameters agree well with associated reference equations of state.