Relativistic correction to the potential energy surface and vibration-rotation levels of water

The relativistic correction to the electronic energy of the water molecule is calculated as a function of geometry using CCSD(T) wavefunctions and first-order perturbation theory applied to the one-electron mass-velocity and Darwin terms. Based on the calculated 324 energy points, a fitted relativistic correction surface is constructed. This surface is used with a high-accuracy ab initio non-relativistic Born–Oppenheimer potential energy surface to calculate the vibrational band origins and rotational term values for H216O. These calculations suggest that the relativistic correction, has a stronger influence on the vibration-rotation levels of water than the Born–Oppenheimer diagonal correction. The effect is particularly marked for vibrational levels with bending excitation or rotational states with high Ka.