Why Calculate the Spectra of Small Molecules?

Calculations on the bound nuclear motion states of molecules are now making a contribution to many areas of physical science including astrophysics, planetary atmospheres and chemical kinetics. This article discusses the various uses made of ro-vibrational calculations on small, particularly triatomic, molecules. A non-technical theoretical overview compares the three main methods for performing these calculations: the traditional perturbation theory approach, basis set methods using the variational principle and a finite-element approach, the discrete variable representation (DVR). The reasons for maintaining parallel basis set and DVR programs are given. Applications of ro-vibrational calculations to the area of traditional high-resolution spectroscopy, i.e. low-lying states, are discussed. These include proving and developing potential-energy surfaces, predicting and assigning spectra, calculating transition intensities and generating data for the calculation of thermodynamic and emissivity parameters. The links between highly excited ro-vibrational states, reaction dynamics and quantum chaology are discussed and the importance for improved calculations in this energy region outlined.