Combined Quantum and Molecular Mechanics (QM/MM) Approaches to Simulate Ultrafast Photodynamics in Biological Systems

Ultrafast photoreactions play a key-role in numerous biological systems. They provide vision, phototaxis, UV-protection as well as energy storage and energy conversion with an efficiency which, so far, has not been reached under laboratory conditions. Detailed mechanistic studies can help to elucidate the underlying principles, but the systems of interest are often too large to be treated solely with accurate quantum methods. This paper reviews current strategies for studying ultrafast biological photodynamics by use of combined QM/MM schemes with surface hopping. In these methods, the photoreactive component is described with a quantum mechanical approach, while surrounding effects provided by solvent molecules or a protein environment are considered through computationally less expensive molecular mechanics techniques. To obtain meaningful results in the simulation of photochemical reactions, the combination of these methods must be carefully balanced. The choice of QM method, the definition of starting conditions for photodynamics simulations, as well as the description of the non-adiabatic event and statistical analysis of ensemble averaged data are critically discussed, highlighting the strengths of the currently applied approaches but also noting common pitfalls.