To quantify the net charge transfer and its change with the electric field Figure shows the net charge transferred from the substrate into the adsorbate (calculated via Mulliken analysis). In agreement with our findings for the charge density, we observe increased charge transfer for positive and decreased charge transfer for negative fields. The effect is stronger for the standing molecule, as can be seen from the slopes of the lines in Figure . In passing, we note that the change in net charge transfer to the adsorbate is primarily due to a change of donation from the substrate into the π-system of the adsorbates, whereas the back-donation of charge from the σ-system of the molecules into the substrate is far less affected by the field (Supporting Information).