When W 3 is placed at a large distance of separation, the interaction energy of the water dimer at the DLPNO-CCSD(T) level is -5.1 kcal/mol, which is consistent with state-of-the-art reference values (see Table ). As W 3 approaches, significantly decreases due to cooperativity effects between W 1 -W 2, W 1 -W 3 and W 2 -W 3 interactions (red line in Fig. ), reaching up to -6.9 kcal/mol. At the same time, the two-body contribution to the binding energy of the dimer frozen at the trimer geometry decreases (blue). The difference between and represent the net contribution of cooperative effects to the W 1 -W 2 interaction. The increase in the water dimer binding strength due to cooperative effects can be explained by the increase in the dipole moment of W 1 and W 2 upon the interaction with W 3 . Our approach allows us to quantify this effect quantitatively at various distances of separation, thus providing a simple theoretical framework in which to analyze cooperativity of intermolecular interactions in complex systems.