A new ab initio three-body potential for water has been generated from the Hartree-Fock method and symmetry-adapted perturbation theory calculations performed at 7533 trimer geometries. The calculated nonadditive energies were then fitted to a physically motivated analytic formula containing representations of short-range exchange contributions and damped induction terms. To our knowledge, this is the first time the short-range nonadditive interactions have been explicitly included in a potential for water. The fitted nonadditive potential was then applied, together with an accurate ab initio pair potential, SAPT-5s, to evaluate the effects of nonadditivity on the structure and energetics of water trimer. (C) 2003 American Institute of Physics.