The results of the first-and second-order exchange corrections calculated within the S 4 approximation generally have much better accuracy than the S 2 approximation. Note, that for the complexes with water molecules, the absolute error of exchange-dispersion energy calculated in S 2 approximation changes its sign in the vicinity of the equilibrium distance (see the exchange-dispersion dips in Figures ). At close intermolecular distances, the E (20) exch-disp (S 2 ) overestimates the value of exchange-dispersion, whereas at further distances tends to underestimate it. A similar behaviour was not observed for the exchange-dispersion . The dip visible in the plot of the exchange-dispersion energy in the S 2 approximation appears because of the difference from the reference value changes its sign.