Role of cancellation of errors in ab initio calculations:: Structure and energetics of the OH- (H2O) system and electric dipole properties of the subsystems

The joint study of the full system and of the electric dipole properties of the subsystems allows an analysis of the role of cancellation of errors on the geometry and energetics of OH-(H2O). Calculations were performed at the Hartree-Fock, MPn (n = 2-4) and CCSD(T) levels. The ab initio dipole moment and static dipole polarizability of OH- and H2O were computed using 15 or so basis sets. Special attention was paid to the delicate case of the polarizability of OH-, which requires much more diffuse polarization functions than for H2O. The MPn series does not converge to the CCSD(T) values. Our recommended values for the parallel and perpendicular polarizability components of OH- are 27.8 and 46.7 au, respectively. For several basis sets, the geometry of the OH-(H2O) system, was optimized, accounting or not for the basis set superposition error (BSSE) in the optimization process. Energetic and geometric data are discussed. From our best calculations, DeltaH (298 K) lies around 26.4-26.7 kcal mol(-1), in excellent agreement with the most recent experimental values of -26.5 +/- 1.0 (Meot-Ner, M.; Speller, C. V. J. Phys. Chem. 1986, 90, 6616). Some bases underestimate the polarizability components of OH-, the effect on the intermolecular energetic contributions being counterbalanced by the BSSE. However, such an "advantageous" balance cannot be guessed "a priori".