First, for systems composed of distinct molecular building blocks such als molecular clusters, we have extended the conventional energy-based many-body expansion (eb-MBE) by a density-based correction, that is derived by performing a manybody expansion of the electron density, and inserting the resulting total density into the total energy functional of density-functional theory (DFT) . This density-based many-body expansion (db-MBE) can be considered as an ONIOM-style multilevel method, in which the high-level is provided by the eb-MBE and the low-level is provided by subsystem-DFT . Previously, we have demonstrated that for water and ionwater clusters, the db-MBE provides accurate and efficient total and relative energies already at the level of a two-body expansion .