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Quantum chemistry of the excited state (QCEX) refers in the context of this chapter to the field of computational chemistry devoted to the research on the chemical transformations involving excited electronic states. Three types of QCEX phenomena can be distinguished:

The QCEX research field is built upon particular “theoretical objects” that are crucial for determining the mechanisms underlying photo-induced and chemically-induced transformations. Those with high relevance for this chapter are briefly defined in the following lines:1 

To get an accurate determination of these objects and a detailed analysis of the electronic structure transformations occurring in excited-state chemistry, the so-called multiconfigurational methods of quantum chemistry (MQC) are optimal. In particular, the complete-active-space second-order perturbation theory (CASPT2) method2–4  shows a good balance between accuracy and computational load. The MOLCAS program5  has a very practical implementation with different flavours of the method (level-shift (LS),6  ionisation potential electron affinity (IPEA) shift,7  multistate (MS),8  extended multistate (XMS),9  extended dynamically weighted (XDW)).10  It is probably one of the most used software packages for this method. BAGEL program11  is a good alternative with highly efficient algorithms for parallel rather than serial processing. MOLCAS interfaces with other programs such as SHARC,12  that allows performing state-of-the-art photodynamic simulations. MOLCAS + TINKER13  allows studies in macromolecules by using quantum chemistry/molecular mechanics (QM/MM) hybrid methods. MOLCAS + COBRAMM + SHARC14,15  has been recently presented to the community by González, Garavelli and co-workers as a promising tool for photodynamics simulations in macromolecules with QM/MM.

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