Quantum chemistry of the excited state: 2005 overview

The present contribution contains an overview of quantum-chemical methods and strategies to compute and interpret spectroscopic and photochemical phenomena in molecular systems. The state of the art for the quantum chemistry of the excited state is reviewed, focusing in the advantages and disadvantages of the most commonly employed computational methods, from the single configurational procedures like CI-Singles (CIS), propagator approaches, and Coupled-Cluster (CC) techniques, to the more sophisticated multiconfigurational treatments, with particular emphasis on perturbation theory, the CASPT2 approach. Also, a short summary on the performance, lights, and shadows of the popular TDDFT methods is included. The role of the differential correlation effects on quantum-chemical calculations is analyzed, especially for the location of potential energy surface crossings. The contribution finally addresses the importance that theoretical constructs as conical and non-conical intersections play in non-adiabatic photochemistry. The nice photochemistry of cytosine is used as an illustrative example of theoretical photochemistry, a continuously expanding field of research.