Credit: Image courtesy of Fournier et al., Journal of Physical Chemistry A 119, 9425 (2015).
Simulations quantify the electric fields (blue) on the hydrogen atom sites using 20 water molecules surrounding a hydronium (H3O+) ion, which is shown on the top of the cluster. The red and green surfaces represent positive (from the nuclei) and negative (from the lone pairs of electrons) voltages acting as local electrodes.