The subjects of contemporary chemical science move towards ever more complex structures and chemical systems . Quantum Chemistry is playing an increasingly important role in the analysis and rationalization of experiments conducted on complex molecules, (bio-)polymers, molecular aggregates or materials, and molecules at interfaces. This trend is reflected in the rapidly growing activity towards quantum-chemical method development for large and complex chemical systems . One attractive approach for the efficient quantum-chemical treatment of such systems are subsystem approaches, which ideally achieve a natural linear scaling with system size by partitioning a large and complex chemical system into smaller fragments .