Kain, Jacqueline Sophie; (2001) Constructing ab initio and empirical potential energy surfaces for water. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The infrared spectrum of water is possibly one of the most well studied and yet portions of it are still poorly understood. Recently, significant advances have been made in assigning water spectra using variational nuclear motion calculations. The major factor determining the accuracy of ro-vibrational spectra of water is the accuracy of the underlying Potential Energy Surface. Even the most accurate ab initio Potential Energy Surface does not reproduce the Born-Oppenheimer surface to sufficient accuracy for spectroscopic studies. Furthermore, effects beyond this model such as the adiabatic correction, the relativistic correction and the non-adiabatic correction have to be considered. This thesis includes a discussion on how the relativistic correction was calculated, for the water molecule, from first-order perturbation theory. The relativistic correction improved vibrational stretching motion while making the prediction of the bending modes far worse. For rotational motion the relativistic effect had an increasing effect with increasing Ka. A further alteration to the ab initio calculations is introduced by adjusting the barrier to linearity in the water potential. This alteration to the barrier was considered in order to compensate for the lack of convergence of quantum chemical calculations of the Born- Oppenheimer surface. This barrier attempts to represent the change in the potential from linear to equilibrium. We show the improvements this has on calculated energy levels by comparison with the HITRAN database. This then led the way to the improved spectroscopic potential presented here in this thesis. This new spectroscopic potential reduces the overall standard deviation significantly for vibrational and rotational energy levels.

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