ABSTRACT
Excited states are of great significance in all of the chemistry initiated by the absorption of light (photochemistry) or by the light-emitting reactions of high-energy systems (chemiluminescence). However, the rapid and reliable description of excited states has been a considerable challenge to computational chemistry. Variational calculations inevitably favor the ground state, and the representation of excited states by excitations from such a favored ground configuration must systematically overestimate excitation energies. In semiempirical approximation, one feels justified in scaling parameters so to match some reference system’s longest-wavelength transition. In ab initio calculations, reasonable treatment of excitations requires so flexible a basis set that it is capable of simultaneous description of the ground state and (at least several low-lying) excited states as well. Despite such difficulties and complications, however, informative calculations are feasible. We will explore in some detail the simplest model chemistry suitable to characterization of low-lying electronically excited states, called configuration interaction (singles) or CIS. We will provide a brief introduction and some guidelines for use of more subtle and demanding methods such as complete active space MCSCF and linear-response techniques.
