As complex processes that they are, the knowledge about the chemiluminescence and bioluminescence phenomena is still being built up. Although much progress was done during the past decades, many mysteries remain unsolved. The mechanism of the complete luciferin-luciferase complex of the firefly is to date not understood in all its details, particularly, the light emitting species is still not conclusively determined. The chemiluminescence mechanism of the luminol molecule is unknown. The aim of this proposal is to apply the unique QM(SA-CASSCF/MS-CASPT2)/MM methodology as recently implemented in the MOLCAS program to understand these processes. Initially, the study of the luciferin-luciferase bioluminescence will be carried out to characterise the mechanism of this process. In a subsequent step the chemical origin of the multicolour bioluminescence will be analysed by applying the QM/MM approach to the wild-type and different mutant luciferin-luciferases. The chemiluminescence mechanism of luminol will be studied after that, by using the multiconfigurational methodology. The results will allow to establish the molecular basis of these processes and characterise them with accuracy, and will be a great impact for accurate studies of photochemical and biochemical processes of biological interest, also contributing to the future development of energy saving technologies and novel analysis processed as in medical applications. By means of the proposed project, the fellow will reach a great expertise in the field of Theoretical and Computational Photochemistry by strengthening his skills in the use of the CASSCF/CASPT2 methodology, acquiring much more experience with the employment of the QM/MM approach, and learning about the MOLCAS source code. The fellowship will allow the applicant to develop a promising scientific career to the European level.