A QM/MD coupling method to model the ion-induced polarization of graphene.

We report a new Quantum Mechanical/Molecular Dynamics (QM/MD) simulation loop to model the coupling between the electron and atom dynamics in solid/liquid interfacial systems. The method can describe simultaneously both the quantum mechanical surface polarizability emerging from the proximity to the electrolyte, and the electrolyte structure and dynamics. In the current set up Density Functional Tight Binding calculations for the electronic structure calculations of the surface are coupled with classical molecular dynamics to simulate the electrolyte solution. The reduced computational cost of the QM part makes the coupling with a classical simulation engine computationally feasible and allows simulation of large systems for hundreds of nanoseconds. We tested the method by simulating a non-charged graphene flake immersed in an NaCl electrolyte solution at varying concentrations. We found that ions preferentially remained in solution and only cations are mildly attracted to the surface of the graphene. This behaviour is found to originate from the relatively small adsorption energy compared to the value of the ion hydration energy and rules out any possible ions/surface charge transfer.