Infrared intensities from the Multimode code

Here we present calculations for infrared intensities using the variational normal coordinate code Multimode. As a working approximation, we only use J=0 vibrational wavefunctions. We use quartic force fields and cubic dipole moment vector surfaces, generated in this instance by density functional theory. We introduce the theory, explain how we evaluate the matrix elements, and give expressions for the intensities. Our examples are water and formaldehyde. In the latter case there are a number of resonances, and the tabular representation is inadequate. Alternatively, we present stick diagrams which show all the contributions to the intensities, including those from components of a resonance. This shows how to represent infrared intensities for larger species.