The calculation of spectroscopic phenomena involving molecular vibrations is an example of a successful application of theoretical chemistry to aid the interpretation of experimental observations. The calculation of molecular structure and vibrational spectra was made possible by the pioneering work of Bratoz, Pulay, Pople, Schaefer and others in developing methods for calculating analytical geometric derivatives of the molecular energy.1–5 Second-order geometric derivatives of the energy have since been derived and implemented for a wide range of correlated wave functions,3,5–13 as well as at the level of density functional theory (DFT).14–16 For a detailed historical account we refer to recent reviews of molecular properties in general and molecular force fields in particular.17–19