Nuclear corrections to molecular properties. V. Refinements in a b i n i t i o normal-coordinate potential energy and property surfaces for water and their effect on the vibrational analysis

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Abstract
A b initio surfaces in reduced normal-coordinate space for water, for the potential energy and components of the dipole moment and second moment of charge about the center of mass, have been refined for use in calculating energies of vibration and expectation values of of these properties in low-lying states of vibration. The two procedural refinements consist of (i) direct fitting of the surfaces to values obtained at a grid of points, where the internal coordinates for each point have been transformed to reduced normal coordinates by an iterative procedure, and (ii) (small) rotation of the axes into coincidence with a vibrationally invariant set at each asymmetric configuration on the grid. The first refinement brings the calculated values of the constants of anharmonicity xij into much closer agreement with experiment. Both refinements generate significant changes in the fitted property surfaces. For several isotopic species of water calculated expansion coefficients are listed for the potential energy in reduced normal coordinates and the dipole moment in internal and reduced normal coordinates; expectation values are also given.