In broad terms, two distinct embedding strategies can be employed. The idea behind the subtractive and additive embeddings is depicted somewhat simplified in Figure . In a subtractive scheme, three separate calculations are performed: One GW-BSE calculation for the isolated subsystem I and calculations at the lower accuracy embedding method (emb), one for the entire system (subsystems I + II) and one for subsystem I. The total energy of the full system is then obtained as