Using a systematic series of basis sets in supermol. and symmetry-adapted intermol. perturbation theory calcns. it is examd. how interaction energies of various water dimer structures change upon addn. and shifting of bond functions. Their addn. to augmented double- and triple-zeta basis sets brings the sum of the electron correlation contributions to the second-order interaction energy nearly to convergence, while accurate first-order electrostatic and exchange contributions require better than augmented quadruple-zeta quality. A scheme which combines the different perturbation energy contributions as computed in different basis subsets performs uniformly well for the various dimer structures. It yields a symmetry-adapted perturbation theory value of -21.08 kJ/mol for the energy of interaction of two vibrationally averaged water mols. compared to -21.29 kJ/mol when the full augmented triple-zeta basis set is used throughout.