Hybrid QM/MM Approach for the Calculation of Excited States in Complex Environments

Understanding photochemistry is crucial to unveil the nature and mechanism behind processes fundamental for life, ranging from photosynthesis, vision, to DNA damage. These processes happen in the presence of an environment, so it is natural to aim to include its effect on the photophysical properties of the active unit in photochemical processes. In this chapter, we want to introduce the key concepts necessary to simulate photophysics and photochemistry in complex environments by means of a hybrid quantum mechanics/molecular mechanics (QM/MM) approach. We will review the theory and the methodology of the QM/MM approach, with an eye always focused on the additional challenges rising from the need of computing electronically excited state (ES) properties with such hybrid scheme. We will support the methodological part with several examples of practical applications, explaining how we included the environmental effects and showing the impact of the surrounding on linear absorption, ESs energies, potential energy surfaces (PESs) shape and photoreactive mechanism. In its totality, we here offer a complete overview on the state-of-the-art methodologies and applications of QM/MM approaches to the calculation of optical properties in the presence of complex environments.