What the previous example highlights is that excitation energies, no matter if charged or neutral, can also be affected by electrostatic effects from the environment that are not merely arising from polarization. Non-symmetrical structural details, for instance, combined with non-negligible electrostatic moments can cause additional shifts of excitation energies with respect to vacuum. Such situations are typically also present in low-dimensional systems, such as surfaces of molecular crystals. Li et al. have extended the approach based on Eq. ( ) to study charged excitations at the (001) surface of a pentacene crystal. In particular, they used the charge response model by Tsiper and Soos to model the polarization effects and combined it with long-range static embedding effects in the low-dimensional environment to account for crystal field effects. Figure shows how the inclusion of different environmental effects affects the calculated ionization potential and electron affinities at the surface. It is very obvious that polarization effects massively reduce the GW gap compared to the gas phase calculation (which agrees favorably with experimental results for gas phase ionization energies). From a purely electrostatic perspective, the excited electron and hole get screened, or energetically stabilized, in the same way, meaning that the ionization potential lowers and the electron affinity increases. Additional inclusion of the crystal field affects both excitation energies in the same direction, i.e., the absolute energies are shifted, but their energy difference is more or less constant.