Potential Energy Surfaces and Vibrational Anharmonicity

Abstract

The central importance of the potential energy surface to the interpretation of molecular spectra is discussed, and methods of determining the PE surface are reviewed. The methods of harmonic force constant calculations for small amplitude vibrations are reviewed, both from the point of view of calculating vibration and vibration-rotation spectra from the force field, and of the inverse calculation in which the force field is determined from spectroscopic observations.

Anharmonicity becomes important as soon as we attempt to calculate overtone spectra, or even for the fundamentals when we look at vibration-rotation effects and large amplitude vibrations. The analysis of anharmonicity is described in terms of a polynomial expansion of the force field and a perturbation treatment of the anharmonic terms. The problems of choosing coordinates and of non-linear coordinate transformations are discussed. Some of the special effects of anharmonicity, such as the transformation from normal mode to local mode behaviour at high energy, are discussed.