Vibration–rotation transition dipoles from first principles

The use of ab initio methods to calculate line positions and associated transition intensities for the infrared spectrum of small molecules has recently become common. The first principles calculation of transition dipoles, upon which the intensity is based, relies on three distinct steps: the quantum chemical calculation of the dipole moment surface at a grid of geometries, the accurate representation of this surface using an appropriate functional form and the wave functions used to represent the initial and final states, which in turn depend on the accuracy of the potential energy surface used to generate them. Each of these stages is discussed with a view to obtaining the highest possible accuracy. The prospect of computed transition intensities displacing measured ones as the primary source of such information is considered.