For the recent q-AQUA, 54 q-AQUA-pol and MB-pol(2023) This approach has also been used for heterogeneous systems, for example clathrate hydrates with different guest molecules such as H 2 , 57 CH 4 , 58 and CO 2 , and notably the hydrated proton. The many-body expansion for covalent interactions for a molecule is less common in the recent literature; however, there is a history using this approach for triatomic and tetraatomic molecules, as developed in the 1980s by Murrell and co-workers. ? A recent example of this approach, relevant to the present work on alkanes, was reported by Paesani and co-workers. ?