The barrier to linearity of water

High-quality ab initio quantum chemical methods, including higher-order cou pled cluster (CC) and many-body perturbation (MP) theory, explicitly correl ated (linear R12) techniques, and full configuration interaction (FCI) benc hmarks, with basis sets ranging from [O/H] [3s2p1d/2s1p] to [8s7p6d5f4g3h2i /7s6p5d4f3g2h] have been employed to obtain the best possible value for the barrier to linearity of water. Attention is given to the degree of accord among extrapolations of conventional MP2, CCSD, and CCSD(T) energies to the complete basis set (CBS) limit and corresponding linear R12 schemes for th ese correlation methods. Small corrections due to one- and two-particle rel ativistic terms, core correlation effects, and the diagonal Born-Oppenheime r correction (DBOC) have been incorporated. The final electronic (vibration less) extrapolated barrier height of this study is 11 127 +/- 35 cm(-1). An harmonic force fields have been determined at the aug-cc-pCVTZ CCSD(T) leve l at equilibrium and at a linear reference geometry. These and previous sex tic force fields are in general accord with the expansion terms of recent g lobal potential energy hypersurfaces but also highlight some of their weakn esses. (C) 1999 American Institute of Physics.