Development of a Transferable Variable Charge Potential for the Study of Energy Conversion Materials FeF2 and FeF3

A variable charge potential is developed that is suitable for the simulations of energy conversion materials FeF2 and FeF3. Molecular dynamics simulations using this potential show that the calculated structural and elastic properties of both FeF2 and FeF3 are in good agreement with experimental data. Such a transferability of this potential rests in the fact that the difference in the bond characteristic between FeF2 and FeF3 is properly accounted for by the variable charge approach. The calculated equilibrium charges are also in excellent agreement with first-principles Bader charges. Surface energies obtained by the variable charge method are closer to the first-principles data than are fixed charge models, indicating the importance of variable charge method for the simulations of the surface. A significant decrease in atomic charges is observed only for the outermost one or two layers, which is also observed in the first-principles calculations.