As an example for GW calculations using periodic embedding in the subtractive sense, we show in Figure the layer-resolved highest-occupied molecular orbital energy diagram of a thin film of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene . The studied film contains about 1000 molecules and the shown values for the respective layers are obtained by averaging over many individual embedded GW calculations. Performing 1000 additive GW -BSE/MM as in Section 3.3 instead is orders of magnitude more computationally expensive. The disadvantage of using the subtractive method is that one needs classical microelectrostatics models for molecules both in their ground state (for the embedding region) and the excited state and that these models provide a sufficiently accurate representation of quantum mechanical electrostatic responses. In Ref. , for instance, this was checked for selected molecules in the bulk and at the surface of the film by comparing results from additive GW /MM and subtractively embedded GW calculations.