It should be noted that the presented theory can also provide insights into interactioninduced changes in density matrices. Naturally, ∆X can be considered as the trace of the property operator X with a change in the density matrix. This matrix can reveal information on the density changes resulting from molecular interactions and their decomposition into physically well-defined contributions in the spirit of SAPT. Given the interpretive power of such an approach, one can relate density changes to specific physical effects. Finally, the theory outlined here can be adapted to include geometric analytical derivatives for SAPT corrections, opening the door to many other applications such as SAPT-based geometry optimizations or direct calculations of frequency shifts in van der Waals complexes. Work on these extensions of the presented theory actively continues in our groups.