Use of a non-pair-additive intermolecular potential function to fit quantum-mechanical data on water molecule interactions Abstract: Permanent and induced multipole, repulsion, and dispersion terms are proposed as a model for the nonadditive potential of water molecules. 229 quantum-mechanical dimer energies of near Hartree-Fock accuracy which have been published by Popkie, Kistenmacher, and Clementi and by Kistenmacher, Lee, Popkie, and Clementi have been fit to an approximation to this model. The standard deviation 0.0019 of calculated dimerization energies for one simple form of the present model compares with a standard deviation 0.0023 given by an additive analytical fit developed by the above group. Nonadditive contributions calculated for the present model are compared with those given by the HF calculations of Hankins, Moskowitz, and Stillinger and by Kistenmacher, Lee, Popkie, and Clementi. The proposed model is compared with other models, and results to aid in the choice of multipole expansion orders in applications are given.