A b i n i t i o SCF and CI studies on the ground state of the water molecule. II. Potential energy and property surfaces

Abstract

Self‐consistent field and configuration interaction calculations for the energy and one‐electron properties of the ground state of the water molecule were carried out with a (5s4p2d/3s1p) 39‐function STO basis set. The CI treatment included all single and double excitation configurations (SD) relative to the SCF configuration, and a simple formula due to Davidson was used to estimate the energy contribution of quadruple excitations and thus produce a set of corrected (SDQ) energies. The calculations were carried out for 36 molecular geometries (22 symmetric and 14 asymmetric) in the neighborhood of the equilibrium geometry. Least‐squares analysis was used to determine theoretical equilibrium geometries and to derive quartic and cubic expansions for the energy and one‐electron properties in terms of internal displacement coordinates. Considerable improvement was found in the SD and particularly in the SDQ results relative to SCF, with the SDQ equilibrium geometry being in excellent agreement with experiment and the harmonic force constants also showing much better agreement than other ab initio results. Results for higher‐order force constants are difficult to assess because of the large uncertainties in both theoretical and experimental results. The dipole moment function obtained in these calculations should be quite useful in the assessment and analysis of infrared intensity data and isotope effects.