Treatment of Electronically Excited States via Coupled-Cluster Theory

Coupled-cluster methods for the accurate calculation of excitation energies in the framework of linear response theory (or alternatively the equation-of-motion ansatz) are reviewed. The basic aspects of coupled-cluster response theory will be described, and the high accuracy obtained in the corresponding treatment of excited states, in particular when triple excitations are included, is demonstrated. Furthermore, recent developments such as analytic derivative techniques for the exploration of excited-state potential energy surfaces, treatment of excitation spectra of radicals via a spin-restricted coupled-cluster ansatz, or the computation of transition moments such as, for example, spin-orbit coupling constants, are discussed.