A brief history of computational photochemistry

Below follows a compact recollection of the development of computational photochemistry during the last 60 years. The list is in no way complete and 

In the early 90s, a second improvement in the field of computational photochemistry took place. The Roos group proposed and implemented the multi-reference second-order perturbation theory (CASPT2), which reduced the CASSCF errors to the level of the accepted chemical accuracy (0.2 eV). This promoted the computational photochemistry to a new stage. Since then, the CASSCF/CASPT2 method is the most general and robust method for qualitative and quantitative accurate studies of any type of photochemical or photophysical process. 

In 1998 a multi-state version of CASPT2, MS-CASPT2, was introduced by Finley et al. to compensate for those cases when the CASSCF reference functions, due to aceidental degeneracy, is not accurate enough to form the basis of state-specific multiconfigurational reference 2nd order perturbation theory.  

5 Hybrid methods for studies of large molecular systems 

Excited-states simulations were mainly limited to small and medium-sized molecules before the 90s. However, many important photophysical processes, as for example, the photoisomerization of rhodopsin, take place in a biological environment, seldom not without the presence of an enzyme. To study photochemical processes in the large-size systems, alternative methods are required. One such method, the QM/MM method, was developed by Warshel and Levitt in 1976. This approach combines the accuracy of quantum chemical models with the speed of molecular mechanics. An alternative method to combine different quantum chemical approaches, the ONIOM method, was developed by Morokuma and co-workers. These methods were initially used in the context of ground-state reactions. Early applications of the QM/MM hybrid method to photochemical processes can be found as early as 1982, however, it was not until at the beginning of this century that the method started to be used extensively for photochemical and photophysical dynamics. To find representative investigations of that time consult the reference list.  

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