Natural division and its gradation

Although the notion of interaction energy is of great practical value, its theoretical meaning is a little bit fuz. Right at the beginning we have a question interaction of what We view the qfstem as composed of particular subsystems, that once isolated, then have to be put together. 

In addition the system might be considered as composed of a hydrogen molecule interacting with two OH radicals  

Choice of subsystems is of no importance from the point of view of mathematics, but is of crucial importance from the point of view of calculations in theoretical chemistry. 

The particular choice of subsystem should depend on the kind of experiment with which we wish to compare our calculations  

Let us stress, that in any case when choosing subsystems we are forced to single out particular atoms belonging to subsystem A and B. It is not sufficient to define the kind of molecules participating in the interaction, see our examples I and II. 

---