The potential profile in the Stern region can be post-calculated using equation ( ) from the values of relative permittivity (ε r,OHP ) and electric field at the OHP obtained from the solution of the GMPNP equations for a given value of potential at the OHP. The width of the Stern layer is assumed to be 0.4 nm which is slightly larger than the radius of the largest cation considered in this study (Li + ). The electric field value and the potential value at the OHP are used as Neumann and Dirichlet boundary conditions for equation (13), respectively.