The use of Cartesian grids in the DFT calculations with localised orbitals and in subsequent processing is expected to subtly break rotational invariance (by distinguishing certain spatial directions corresponding to grid edges) and translational invariance (so-called eggbox effect). These effects are minimised in ONETEP where the local orbitals (NGWFs) are optimised in situ, but they are not completely eliminated. To alleviate any concerns over whether these have measurable effect on the obtained electric fields, we performed six additional calculations for one of the MM frames, where the systems were variously rotated and translated relative to the grid. Typical error magnitudes were found to be in the order of 0.1 MV/cm, with a maximum below 0.4 MV/cm, making them negligible in our analysis.