Approximate molecular orbital anharmonic parameters for the infrared spectrum of H2O.

The vibrational spectrum of H2O was calculated at MP2/6-31G(extended) and MP2/6-311G* levels taking into account anharmonicities through a simple approach to second-order perturbation theory in which molecular energy and dipole moment are expanded as Taylor series in normal coordinates with no cross terms, to simplify calculations. The series coefficients are obtained separately for each normal coordinate through polynomial regression of calculated single point property values corresponding to a few distorted molecular geometries. The energy coefficients are used to calculate the harmonic frequencies and the chi(ii) anharmonicity constants, and so the band origins. For the band intensities, second-order perturbation theory equations derived earlier for diatomic molecules are used for each mode. Estimated frequencies have accuracy equivalent to those of previous complete perturbation calculations at the same ab initio levels, being at most 2.6% above the experimental values for the MP2/6-31G(extended) level. The fundamental intensity estimates are equivalent to those for the complete treatments, with the exception of that at MP2/6-31G(extended) level for the bending mode, which is 7% above the experimental value. Estimated overtone intensities by both complete treatments and the simple approach may still differ in magnitude from the experimental values, though to a lesser extent for the formers.