Electronic structure, Raman tensors, and resonance phenomena in a simple molecular model

Some concepts in modern spectroscopy are very specialized, and explanations based on simple examples are not readily available. An example is the changes in the intrinsic symmetry of Raman tensors in molecules produced by resonance or near-resonance conditions. Many of these effects can be obtained from commercial and open source programs that solve the electronic structure of the molecules with density functional theory and compute the Raman tensors of the vibrations. The origin of these changes is hidden by the complexity of these calculations and by the many intermediate computational steps that are not presented to the user. We discuss a simplified model for the electronic structure of a molecule and correlate what is observed with a calculation using density functional theory for a specific molecule. The model yields insight into resonance phenomena, symmetry-related aspects of Raman tensors, and the microscopic origin of the Raman effect itself.