To calculate electric fields, we first calculated electrostatic potentials on a Cartesian grid with a spacing of about 0.13 Å (a so-called double grid, with a spacing sufficiently fine to represent densities). Two separate components to the potentials were considered -one due to the valence-electronic pseudodensity (Hartree potential), and one due to the local pseudopotential. Both components were obtained in reciprocal space using Fast Fourier Transforms (FFTs) using standard ONETEP methodology. Exchange and correlation potentials, representing non-classical effects, were excluded from analysis. Electric fields were obtained from the potential in reciprocal space, where the gradient operator is simply 𝑖𝑔 ⃗: