The infrared absorption intensities of the water molecule: A quantum chemical study

Abstract

Using ab initio Hartree–Fock SCF, configuration interaction, and approximate coupled pair techniques, an extensive quantum chemical study of the infrared absorption intensities of the water molecule is reported. In an attempt to establish the necessary requirements for the quantitative prediction of infrared intensities four major effects were examined: (a) the magnitude of electron correlation effects, (b) the applicability of the Hellmann–Feynman theorem to the calculation of dipole moments functions, (c) basis set requirements, and (d) the accuracy of the customary harmonic approximations. The calculated infrared intensities for the water molecule and several of its isotopic derivatives are in good agreement with the available experimental data. In addition to the fundamentals the intensities of several overtone and combination bands were also calculated resulting in generally very small values that strongly depend on the potential energy surface used in the generation of vibrational wave functions.