The ground-state potential energy surface of water: barrier to linearity and its effect on the vibration–rotation levels

The major factor determining the accuracy of ro-vibrational spectra of the water molecule is the accuracy of the underlying potential energy surface (PES). We discuss improving the ab initio PES by introducing a correction to represent, accurately, the change in potential from equilibrium to linear geometries. We show the improvements which this has on calculated vibrational band origins and rotational energy levels by comparison with experimental data. We predict a barrier to linearity of 11 105±5 cm−1, consistent with, but more accurate than, recent studies. This potential provides the optimum starting point for a proper fit of the PES.